While at first the mines were worked, like quarries, from the surface, and while the great Premier mine is still so worked, most of the present mines are worked by sinking shafts in the native rock outside of the blue ground and then tunneling into the diamond-bearing rock laterally, removing it to the surface, allowing it to weather on the "floors" until it crumbles, then crushing and washing it and concentrating the heavy minerals by gravity methods. Large diamonds are then picked out of the concentrates by hand and small ones and fragments are removed by the "greasers," which are shaking tables heavily smeared with grease over which the concentrates are washed and to which diamond alone, of all the minerals in the concentrate, sticks. The grease is periodically removed and melted, and the diamonds secured. The grease can then be used again.

German South West Africa furnishes a considerable output of very small diamonds, which are found in dry sand far from any present rivers. These diamonds cut to splendid white melee and the output is large enough to make some difference in the relative price of small stones as compared to large ones. The South West African field seldom yields a stone that will afford a finished quarter-carat diamond.

RUBIES. Passing on to the occurrence of the corundum gems we will consider first the ruby. Most fine rubies come from Burmah. The district in which they are found is near Mogok. Practically all the fine pigeon-blood rubies come from this district. The fashion for red stones being for the time little in evidence rubies are not now in great demand. This cessation of demand can hardly be laid to the competition of the scientific ruby, for the sapphire is now very much in vogue, yet scientific sapphires resemble the natural ones even more closely than do the rubies.

Siam furnishes a considerable number of dark garnet-like rubies. These do not command high prices. They are, however, sometimes very beautiful, especially when well cut for brilliancy, and when in a strong light.

Ceylon furnishes a few rubies and a few red corundums have been found in North Carolina.

The Burmese rubies appear to have been formed in a limestone matrix, but most of those obtained are gotten from the stream beds, where they have been carried by water after weathering out from the mother rock.

The rubies of Ceylon, too, probably originated in a limestone matrix, but are sought in stream gravels.

SAPPHIRES. Fine blue sapphires originate in Siam in larger numbers than in any other locality. Kashmir, in India, also supplies splendid specimens of large size. Ceylon, too, furnishes a good deal of sapphire, but mostly of a lighter color than the Kashmir sapphire. The Ceylon sapphires are found in the streams, but originate in rock of igneous origin.

Montana furnishes considerable quantities of sapphire, some of which is of very good color. It is, of course, as good as the Oriental if of equal color, being of the same material. The better colored sapphire from Montana is mined from the rock. Most of the sapphires found in the river gravels near Helena, Mont., are greenish blue or of other colors, and not of fine blue.

Queensland and Victoria in Australia supply considerable quantities of sapphire. When blue the Australian sapphire is usually too dark to be very valuable. The golden and other "fancy" sapphires of the trade come largely from the Ceylon gravels. Siam yields silky brown stones and some fine green ones. Some of the Australian sapphires when cut in certain directions yield green stones.

CHRYSOBERYL. Chrysoberyl of the variety Alexandrite now comes mainly from Ceylon, although formerly from the Ural Mountains.

The cat's-eyes also come chiefly from Ceylon.

The yellowish-green chrysoberyls (which jewelers sometimes call chrysolite) come both from Ceylon and from Brazil. They are frequently found in papers of "fancy sapphires" or "fancy color stones," so called.

SPINEL. Spinels are found along with ruby in Burmah and in Siam and they also occur in the gem gravels of Ceylon. Limestone is the usual matrix of spinel, although it is more often mined in gravels resulting from the weathering of the matrix.

TOPAZ. True topaz, of wine-yellow color, comes mostly from Brazil. Ceylon also furnishes yellow topaz. Asiatic Russia furnishes fine large blue or blue-green crystals resembling aquamarine in appearance. Most of the topaz found in other localities is pale or colorless. Several of our western States, notably Utah, Colorado, and California, furnish colorless topaz. Mexico and Japan also produce it. It is seldom cut, for, while producing a rather brilliant stone, it has little "fire" and is therefore not very attractive.

EMERALD AND AQUAMARINE. Beryl of the emerald variety is exceedingly scarce in the earth. Most of the best emerald comes from Colombia, South America. Large crystals of paler color come from the Urals.

Like ruby and spinel, emerald usually originates in limestone. One is tempted to suspect that these stones are of aqueous origin and that sapphires, and beryl, other than emerald, are more likely of igneous origin.

Beryls of the aquamarine type occur in many places, but usually of too pale a tint, or too imperfect, to be worthy of cutting. Fine gem beryl of blue and blue-green tints comes from Siberia and from several places in the Ural Mountains on their Asiatic slopes.

The Minas Geraes district of Brazil, famous for all kinds of gem stones, furnishes most of the aquamarine of commerce. The pegmatite dikes of Haddam Neck, Conn., of Stoneham, Me., and of San Diego County, Cal., have furnished splendid aquamarine and other beryl. These dikes, according to the geological evidence, are the result of the combined action of heat and water. Thus both melting and dissolving went on together and as a result many fine gem minerals of magnificent crystallization were formed during the subsequent cooling. The longer the cooling lasted and the more free space for growth the crystals had, the larger and more perfect they got. The author has himself obtained finely crystallized aquamarine and tourmaline from the Haddam, Conn., locality and the best specimens there occur in "pockets" or cavities in the coarse granite. Within, these pockets are lined with crystals of smoky quartz, tourmaline, beryl, and other minerals. Sometimes crystals occur in mud or clay masses inside the cavities and such crystals, having been free to grow uninterruptedly in every direction, were perfect in form, being doubly terminated, and not attached anywhere to the rock.

Madagascar has in recent years furnished the finest pink beryl, which has been named Morganite. Yellow beryl (Heliodor) and aquamarine also occur in Madagascar.

ZIRCON. Zircon comes on the market mainly from Ceylon. It deserves to be as much esteemed in this country as it is in Ceylon, for its optical properties are such that it is a very snappy stone. Some of the colors in which it occurs, such as the golden browns, lend themselves nicely to the matching of gems and garments, and, with the growth of education in such matters, jewelers would do well to get better acquainted with the possibilities of zircon and to introduce it to their customers. The supply from Ceylon is sufficient to justify popularizing the stone. Small zircons are found in almost every heavy concentrate, as, for example, in the concentrates of the diamond mines of South Africa, and in those of gold placers in many places. The rough stones resemble rough diamonds in luster and are sometimes mistaken for diamonds.

GARNETS. Garnets of various types are found widely distributed in nature. Perhaps the Bohemian supply is best known, having furnished a host of small stones which have usually been rose cut for cluster work or made into beads. The Bohemian garnets are of the pyrope or fire-red type. Relatively few large stones of sufficient transparency for cutting are produced in the Bohemian mines. The so-called "Cape rubies" of the diamond mines of South Africa are pyrope garnets and some large and fine ones are found. The "Arizona rubies" are pyrope garnets, and while seldom of notable size, some are of very fine color, approaching deep rubies, and the color remains attractive by artificial light.

Almandite garnet, the "almandine" of the jeweler is less abundant than pyrope, when of gem quality. Ceylon furnishes some and India furnishes perhaps more. Brazil, from its prolific gem gravels at Minas Novas, supplies good almandite, and smaller quantities are found in many different localities.

Hessonite garnet, the cinnamon stone or "hyacinth" (incorrect) of the trade, comes mainly from Ceylon.

Andradite garnet, of the variety known as demantoid, from its diamond-like properties, and which is usually sold under the misleading name "olivine" in the trade, comes from the western slopes of the Ural Mountains.

TOURMALINE. Gem tourmaline comes from Ceylon, from Madagascar, from the Ural Mountains, from Brazil, from Maine, from Connecticut, and from California.

The Ceylon tourmalines are mostly yellow or yellowish green, sometimes fine olive-green. Those from the Urals may be pink, blue or green. Brazilian tourmalines are usually green, but sometimes red. In fact in many localities several colors of tourmaline are usually found together and it may be that a single crystal will be green in most of its length but red or pink tipped. Some crystals have a pink core and a green exterior. The author has found both of the two latter types in the Haddam, Conn., tourmalines, and on one occasion was surprised to get back a wine-colored tourmaline from a cutter to whom he had sent a green crystal. There was but a thin shell of the green material on the outside of the crystal.

Some of the Madagascar tourmaline is of a fine brownish red, almost as deep as a light garnet, and much clearer than most garnet.

Would it not be fitting on account of its occurrence in several localities in the United States, for Americans to use more tourmaline in their jewels? The quality of some of the tourmalines of Maine, and of California especially, is not excelled by tourmaline from any other locality. Some of the Maine tourmaline is of a delightful, slightly bluish-green tint that almost approaches emerald.

KUNZITE. Spodumene, of the variety kunzite, comes from San Diego County, California.

QUARTZ GEMS. Coming now to the quartz gems we find amethyst and citrine, or golden quartz widely distributed so that only the localities that furnish the better grades of these stones need be mentioned. Siberia and Uruguay furnish fine amethyst. Brazil also furnishes large quantities of very good quality.

AMETHYST. The chief charm of the Siberian amethyst lies in its large red component, which enables it to change from a deep grape-purple by daylight to a fine red by artificial light that is rich in red rays, and poor in blue ones. The paler types of amethysts that were once esteemed, probably for lack of the rich deep variety, become gray in appearance and much less lovely under artificial light. India furnishes some amethysts, and papers of "fancy color stones" containing native cut gems from Ceylon, frequently contain amethysts, but Brazil, Uruguay, and Siberia furnish the great bulk of the stones that are regarded as choice to-day.

YELLOW QUARTZ. Citrine or golden quartz comes mainly from Brazil. The "Spanish topaz" is sometimes the result of heating smoky quartz from Cordova province in Spain. Our own western mountains furnish considerable yellow and smoky quartz fit for cutting.

ROSE QUARTZ. Rose quartz of the finest quality comes from South Dakota. Bavaria, the Ural Mountains, and Paris, Maine, have also furnished it.

AGATE. Agates of the finest types, such as carnelian and sard, come principally from Brazil and from India.

OPAL. Opals now come most largely from Australia, the Hungarian mines yielding but few stones at present. The fine black opals of New South Wales are unsurpassed by any that have ever been found elsewhere. Mexico furnishes considerable opal, and is notable for its fine "fire opal" or "cherry opal."

JADE. Most of the jade of the variety nephrite that is obtained to-day comes from several of the provinces of China or from Siberia or from Turkestan. A dark-green nephrite comes from New Zealand.

Jade of the jadeite variety, which is harder than nephrite and more highly valued, is rare. The best specimens come from Upper Burmah. It is also found in China and in Tibet.

PERIDOT. Peridot, and the brighter olivine or chrysolite, while of the same mineral species, do not seem to occur together. The darker bottle-green specimens come from the Island of St. John in the Red Sea. It is said that many of the finer peridots now available have been recut from old stones mined many years ago.

Queensland supplies light-green chrysolite, and Arizona a yellowish-green variety. Light-green stones have been found near the ruby mines of Upper Burmah.

MOONSTONE. Moonstone comes mainly from Ceylon. The native cut specimens are sent here and recut, as, when native cut, the direction of the grain is seldom correct to produce the moonlight effect in symmetrical fashion. The native cutters apparently try to retain all the size and weight that is possible, regardless of the effect.

TURQUOISE. Turquoise of the finest blue and most compact texture (and hence least subject to color change) comes from the province of Khorasan in Persia. Several of our western states supply turquoise of fair quality, notably New Mexico, Arizona, Nevada, and California.

LAPIS LAZULI. Lapis Lazuli comes from Afghanistan, from Siberia, and from South America.

MALACHITE. Malachite is found in many copper mines, but principally in those of the Ural Mountains.

AZURITE. Azurite is found in the Arizona mines and in Chessy, in France (hence the name chessylite, sometimes used instead of azurite).

* * * * *

REFERENCES. Students who wish to get a fuller account of the occurrence of precious stones should run through G. F. Herbert-Smith's Gem-Stones under the different varieties. This work is the most recent authentic work of a strictly scientific character. Dr. George F. Kunz's Gems and Precious Stones of North America gives a detailed account of all the finds in North America up to the time of publication. Many of these are of course of little commercial importance. The Mineral Resources of the United States contains annually a long account of the occurrences of gem materials in this country. A separate pamphlet containing only the gem portion can be had gratis from the office of the United States Geological Survey, Washington, D. C.



LESSON XXII

HOW ROUGH PRECIOUS STONES ARE CUT

ROUGH PRECIOUS STONES. John Ruskin, who had the means to acquire some very fine natural specimens of gem material was of the opinion that man ought not to tamper with the wonderful crystals of nature, but that rather they should be admired in the rough. While one can understand Ruskin's viewpoint, nevertheless the art of man can make use of the optical properties of transparent minerals, properties no less wonderful than those exhibited in crystallization, and indeed intimately associated with the latter, and, by shaping the rough material in accordance with these optical properties, greatly enhance the beauty of the gem.

No material illustrates the wonderful improvement that may be brought about by cutting and polishing better than diamond. In the rough the diamond is less attractive in appearance than rock crystal. G. F. Herbert-Smith likens its appearance to that of soda crystals. Another author likens it to gum arabic. The surface of the rough diamond is usually ridged by the overlapping of minute layers or strata of the material so that one cannot look into the clear interior any more than one can look into a bank, through the prism-glass windows that are so much used to diffuse the light that enters by means of them. Being thus of a rough exterior the uncut diamond shows none of the snap and fire which are developed by proper cutting.

As the diamond perhaps shows more improvement on being cut than any other stone, and as the art of cutting the diamond is distinct from that of cutting other precious stones, both in the method of cutting and in the fact that the workers who cut diamonds cut no other precious stones, it will be well to consider diamond cutting separately.

Before discussing the methods by which the shaping and polishing are accomplished let us consider briefly the object that is in view in thus altering the shape and smoothing the surface of the rough material.

HOW CUTTING INCREASES BRILLIANCY. Primarily the object of cutting a diamond is to make it more brilliant. So true is this that the usual form to which diamonds are cut has come to be called the brilliant. The adjective has become a noun. The increased brilliancy is due mainly to two effects: First, greatly increased reflection of light, and second, dispersion of light. The reflection is partly external but principally internal.

Taking up first the internal reflection which is responsible for most of the white brilliancy of the cut stone we must note that it is a fact that light that is passing through any transparent material will, upon arriving at any polished surface, either penetrate and emerge or else it will be reflected within the material, depending upon the angle at which the light strikes the surface. For each material there is a definite angle outside of which light that is passing as above described, is totally reflected within the material.



TOTAL REFLECTION. For diamond this critical angle, as it is called, is very nearly 24 deg. from a perpendicular to the surface. If now, we shape a diamond so that most of the light that enters it from the front falls upon the first back surface that it meets, at an angle greater than 24 deg. to a perpendicular to that surface, the light will be totally reflected within the stone. The angle at which it is reflected will be the same as that at which it meets the surface. In other words the angles of incidence and of reflection are equal. See Fig. 9 for an illustration of this point.

THEORY OF THE "BRILLIANT." In the usual "brilliant" much of the light that enters through the front surface is thus totally reflected from the first rear facet that it meets and then proceeds across the stone to be again totally reflected from the opposite side of the brilliant. This time the light proceeds toward the top of the stone. See Fig. 10—(From G. F. Herbert-Smith's Gem-Stones).

The angles of the top of a brilliant are purposely made so flat that the up coming light fails to be totally reflected again and is allowed to emerge to dazzle the beholder. In the better made brilliants the angle that the back slope makes with the plane of the girdle is very nearly 41 deg. and the top angle, or angle of the front slope to the plane of the girdle is about 35 deg.. Such well made brilliants when held up to a bright light appear almost black—that is, they fail to pass any of the light through them (except through the tiny culet, which, being parallel to the table above, passes light that comes straight down to it).



In other words, instead of allowing the light to penetrate them, well-made brilliants almost totally reflect it back toward its source, that is, toward the front of the stone. The well-cut diamond is a very brilliant object, viewed from the front.

We must now consider how the "fire" or prismatic color play is produced, for it is even more upon the display of fire than upon its pure white brilliancy that the beauty of a diamond depends.

CAUSE OF "FIRE." As we saw in Lesson X. (which it would be well to re-read at this time), white light that changes its course from one transparent medium to another at any but a right angle to the surface involved, is not only refracted (as we saw in Lesson II.) but is dispersed, that is, light of different colors is bent by differing amounts and thus we have a separation of the various colors. If this takes place as the ray of light leaves the upper surface of a brilliant the observer upon whose eye the light falls will see either the red, or the yellow, or the blue, as the case may be, rather than the white light which entered the stone. If instead, the dispersion takes place as the light enters the brilliant the various colored rays thus produced will be totally reflected back to the observer (slightly weakened by spreading, as compared to the direct or unreflected spectra). Thus dispersion produces the "fire" in a brilliant.

Other materials than diamond behave similarly, but usually to a much smaller extent, for few gem materials have so high a refractive power or so great a dispersive power as diamond.

Having considered the theory of the brilliant we may now take up a study of the methods by which the exceedingly hard rough diamond is shaped and polished.

CLEAVING DIAMONDS. If the rough material is of poor shape, or if it has conspicuous defects in it which prevent its being made into a single stone, it is cleaved (i. e., split along its grain). Hard as it is, diamond splits readily in certain definite directions (parallel to any of the triangular faces of the octahedral crystal). The cleaver has to know the grain of rough diamonds from the external appearance, even when the crystals, as found, are complicated modifications of the simple crystal form. He can thus take advantage of the cleavage to speedily reduce the rough material in size and shape to suit the necessity of the case. The cleaving is accomplished by making a nick or groove in the surface of the rough material at the proper point (the stone being held by a tenacious wax, in the end of a holder, placed upright in a firm support). A thin steel knife blade is then inserted in the nick and a sharp light blow struck upon the back of the knife blade. The diamond then readily splits.

"CUTTING DIAMONDS." The next step is to give the rough material a shape closely similar to that of the finished brilliant but rough and without facets. This shaping or "cutting" as it is technically called, is done by placing the rough stone in the end of a holder by means of a tough cement and then rotating holder and stone in a lathe-like machine. Another rough diamond (sometimes a piece of bort, unfit for cutting, and sometimes a piece of material of good quality which it is necessary to reduce in size or alter in shape) is cemented into another holder and held against the surface of the rotating diamond. The holder is steadied against a firm support. It now becomes a case of "diamond cut diamond," each stone wearing away the other and being worn away itself.

The cutting process is fairly rapid and it leaves the stone (which is reversed to make the opposite side) round in form and with a rounding top and cone-shaped back. Stones of fancy shape, such as square, or cushion shape, have to be formed in part by hand rubbing or "bruting" as it is called.

The facets must now be polished onto the stone. Usually the workers who cut do not cleave or polish.

"POLISHING" DIAMONDS. The polisher fixes the cut stone firmly in a metallic holder called a dop, which is cleverly designed to hold the stone with much of one side of it exposed. The holder is then inverted so that the stone is beneath and a stout copper wire attached to the holder is then clamped firmly in a sort of movable vise. The latter is then placed on the bench in such a position that the diamond rests upon the surface of a rapidly revolving horizontal iron wheel or "lap" as it is called. The surface of the latter is "charged" with diamond dust, that is, diamond dust has been pushed into the metal surface which thus acts as a support to the dust. The latter wears away the diamond, producing a flat facet. The lap is kept moistened with oil and from time to time fresh oil and diamond dust are applied. A speed of about 2,000 rotations per minute is used.

FACETTING. The making of the facets is rather slow work, especially when, as is usually the case in making the "table" the work has to be done against one of the "hard points" of the crystal. Great care has to be taken to place the stone so that the grain lies in a correct position, for diamond cannot be polished against the grain, nor even exactly with it, but only obliquely across it. This requirement, as much as anything, has prevented the use of machines in polishing diamonds. The table is usually first polished on, then the four top slopes, dividing the top surface into quarters, then each of the four ridges thus left, is flattened, making eight facets and finally 32 facets, exclusive of the table, are made upon the top of the brilliant. The stone is then reversed and 24 facets, and the culet, polished on the back. As each facet nears its proper shape the stone is placed upon a particularly smooth part of the lap and a slight vibratory motion given to the holder by the hand. This smooths out any lines or grooves that may have formed because of inequalities of surface of the lap. When completely facetted the brilliant is finished and requires only to be cleaned, when it is ready for sale.



LESSON XXIII

HOW ROUGH PRECIOUS STONES ARE CUT AND WHAT CONSTITUTES GOOD "MAKE"—Concluded

SLITTING AND CLEAVING. The cutting and polishing of precious stones other than diamond is a trade entirely distinct from diamond cutting. The precious stone lapidary cuts every species of stone except diamond. The methods used by different lapidaries vary somewhat in their details, and there are many trade secrets which are more or less jealously guarded by their possessors, but in general the methods used to reduce the rough materials to the finished gems are as follows: First, the rough material, if of too large size, or if very imperfect, is slitted, or, if it possesses a pronounced cleavage, it may be cleaved, in order to reduce the size or to remove imperfect parts. Slitting is accomplished by means of a circular disc of thin metal which is hammered so that it will be flat and rotate truly, and is then clamped between face plates, much as an emery wheel is held. The smooth edge of the circular disc is then charged with diamond dust and oil, the diamond dust being bedded into the edge of the metal disc by the pressure of some hard, fine-grained material, such as chalcedony, or rolled into the metal by the use of a rotating roller. Once charged, and kept freely supplied with oil, a slitting wheel will slice a considerable number of pieces of any precious stone less hard than diamond, and will do so with considerable rapidity. The wheel is, of course, rotated very rapidly for this purpose.

The cleaving of certain gem materials, such as true topaz (which splits perfectly across the prism, parallel to its base) is easily accomplished, and it is done in much the same manner as the cleaving of diamond. The feldspar gems, such as moonstone, amazonite, and labradorite, also cleave very smoothly in certain directions. Spodumene, of which Kunzite is a variety, cleaves almost too easily to be durable. Most gem minerals, however, lack such perfect cleavage and when it is desired to remove imperfect parts, or to reduce large pieces to smaller sizes, these materials are slitted as above described.

"RUBBING DOWN." The material being of nearly the dimensions of the finished piece, the next step is to "rub it down," as it is called, to approximately the shape and size desired. This rubbing down process was formerly done by means of a soft metal lap (sometimes of lead), charged with coarse emery powder and water. Carborundum, being harder and sharper than emery, has replaced it very largely. Some of the softer materials, such, for example, as turquoise, are rubbed down on a fast flying carborundum wheel of similar type to those used in machine shops for grinding steel tools. These wheels rotate in a vertical plane and are kept wet. The laps before mentioned run horizontally. The carborundum wheels have the grains of carborundum cemented together by means of some binding material and this gradually crumbles, exposing fresh, sharp cutting edges. Various sizes of grain, and various degrees of hardness of the binding material, as well as various speeds, are needed to suit the many different materials rubbed down by the lapidary. Some lapidaries rub down the harder and more valuable gems such as ruby upon diamond charged laps of brass or other metal.

CABOCHONS. The rubbing down process does not leave a facetted surface, but only a coarse roughly rounded or flattened surface. If the material is to be left in some one of the flat-backed, rounded top forms known as cabochon cut, the surfaces need only to be smoothed (by means of very fine abrasives such as fine emery applied by means of laps, or even by fine emery or carborundum cloth), and they are then ready for polishing.

FACETTED STONES. If, however, the stone is to be facetted in either the brilliant form, somewhat like the diamond, or step cut or otherwise facetted, it is cemented strongly onto a holder (much like the wooden part of a pen holder). The upper end of the holder is rested in one of a series of holes in what is called a "ginpeg" resting in the work-bench near a metal lap, and the stone is pressed upon the rapidly rotating surface of the lap, which is charged with diamond dust or carborundum, according to the hardness of the material to be facetted. A flat facet is thus ground upon the stone. By rotating the holder a series of facets, all in the same set, is produced. The holder is then changed to a new position on the ginpeg and another set of facets laid upon the stone. Thus as many as four or five tiers or sets of facets may be applied to one side, say the top of the stone. The latter is then removed from the holder and cemented to it again, this time with the bottom exposed, and several sets of facets applied.

The stone is now cut but not polished. The facets are flat, but have a rough ground-glass like surface. The polishing is usually done by workers who do not cut stones, but who do nothing but polish them. In small shops, however, the same lapidary performs all the parts of the work.

POLISHING. The polishing of stones, whether cabochon or facetted, is accomplished by the use of very finely powdered abrasives such as corundum powder, tripoli, pumice, putty powder, etc. Each gem material requires special treatment to obtain the best results. It is here that most of the trade secrets apply.

The troubles of the lapidary in getting the keen polish that is so much admired on fine gems are many. In general, the polishing powder should not be quite as hard as the material to be polished, else it may grind rather than polish. The material should be used with water or oil to give it a creamy consistency. It should be backed by laps of different materials for different purposes. Thus, when backed by a fairly hard metal even tripoli, although much softer, will polish sapphire. On a lap of wood, tripoli would fail to polish hard materials, but would polish amethyst or other quartz gem. A change of speed of the lap, too, changes the effect of the polishing material. I have seen a lapidary, who was having no success at polishing an emerald, get very good results by using a stick as a brake and slowing down his lap.

The polishing material must be of very uniform size, preferably water floated or oil floated, to give good results. The lap must be kept flat and true and the stone must be properly held, or the flatness of the facets, upon which brilliancy depends in part, will be destroyed during the polishing.

The softer materials, such as opal, require treatment more like that accorded cut glass, and soft abrasive powders, such as pumice, suffice to polish them. Probably hardly two lapidaries would work exactly alike in their treatment of precious stones, and each guards his secrets, yet all use approximately similar general methods. Some have devised mechanical holders which permit the repeated cutting of stones to exactly the same angles, and that, too, with an accurate knowledge of the angles used. These angles can be definitely altered for different materials, according to their refractive indices. Other lapidaries produce very fine results by purely hand methods.

These details have been gone into to give an idea of the methods of the lapidary and of the many variations in method. In general, however, the slitting or cleaving, the rubbing down to shape, the smoothing out of all scratches and the facetting and polishing are done somewhat similarly by all lapidaries.

Having now had a glimpse of the methods of the lapidaries, let us briefly consider what constitutes good "make" in stones other than diamond.

GOOD "MAKE" IN COLORED STONES. Brilliants, cut from materials having smaller refractive indices than diamond, (and this group includes nearly all stones other than diamonds) should have steeper back angles and higher tops than the best diamond brilliants have. A 35-degree top angle (the angle between the slope of the top and the plane of the girdle is called the top angle) and a 41-degree back angle being about ideal for diamond, other gem materials should have more nearly a 39-degree top angle and a 44-degree back angle to give the greatest possible brilliancy. However, in the case of colored gems such as ruby, sapphire, etc., where the value depends even more largely upon the color than upon the brilliancy, it is frequently necessary to cut the brilliant thicker or thinner than these proportions in order to deepen or to thin the color.

In general, the thicker a stone of a given spread the deeper the color will be. The color may also be deepened by giving to the stone a rounded contour, both above and below the girdle, and facetting it in steps instead of in the brilliant form. Increasing the number of steps also serves to slightly deepen the color, as a larger number of reflections is thus obtained within the material, the light thus has to travel a greater distance through the colored mass, and more of the light, of color other than that of the stone, is absorbed.

IMPROVING COLOR BY PROPER CUTTING. In addition to the color improvement that can be brought about by changing the shape of the cut stone there are a number of gem materials whose color varies very greatly in different directions, and this fact calls for skillful use in order to obtain the best possible results. Thus most tourmalines of deep color must be cut with the top or table, of the finished stone, on the side of the prismatic crystal rather than at right angles to the axis of the prism. If cut the latter way they would be much too dense in color. On the other hand, most blue sapphires should be cut across the prism axis rather than the way that tourmalines should be cut. To cut a sapphire with its table on the side of the prism would be likely to cause it to have a greenish cast because of the admixture of the unpleasing "ordinary ray" of yellowish tint with the blue of the stone as seen up and down the prism. Some Australian sapphires are of a pronounced green when viewed across the axis of the crystal.

Rubies if cut, as was recommended for sapphires, give a very pure and very deep red color, but lack somewhat in the display of dichroism given by rubies that are cut with the table on the side of the crystal and parallel to its axis. Lapidaries need to know and to make use of such optical relations as these and jewelers might well inform themselves in such matters, especially if they have, or hope to acquire, trade in very fine colored stones.

EFFECT OF SHAPE ON BRILLIANCY. In actual practice it is common to find colored stones poorly cut for brilliancy, especially central brilliancy, and that, too, without the excuse of sacrifice of brilliancy in order to improve color. The fault is usually due to too great a desire to save size and weight. Frequently a stone would have greater value if properly cut, even at the expense of some size and weight. When stones are cut too shallow, as is frequently the case, they are sure to leak light in the center and they are thus weak and less brilliant there than they would be if made smaller in diameter and with steeper back slopes approximating 44 degrees.

Round stones, if their angles are correct, are more brilliant than stones of other contour such as square or cushion shape, or navette or heart shape. It can readily be seen that such odd-shaped stones can hardly have the same top and back angles at every part of their circumference. If the angle from a corner of a square stone is correct then the angle from the middle of one side is obviously a little different. Small differences of angle make considerable differences in the brilliancy of cut stones. The prevailing tendency to cut nearly all diamonds round depends largely upon the above facts. In the case of colored stones, however, the added attractiveness which comes with odd or different contour more than makes up for the slight loss of brilliancy that may attend upon the shape selected. Such shapes as lend themselves to special designs in mountings also justify any little loss in brilliancy that accompanies the change in shape, provided the proportions retained give a considerable amount of total reflection within the stone and thus light up most of the stone as seen from the front.

The test of the "make" of a color stone is its appearance. If it lights up well over most of its surface and if the color is right, one should not criticize the "make" as one would be justified in doing in the case of a diamond. If, however, the effect is less attractive it would many times be advisable to measure the angles of the stone, or its thickness and spread as compared with similar measurements on a stone of fine appearance. Frequently one will thus find the reason for the failure of the stone to perform as it might, and recutting should be resorted to in such cases in order to get a smaller but more beautiful and hence more valuable stone.



LESSON XXIV

FORMS GIVEN TO PRECIOUS STONES

While precious stones are cut to many different forms, there are, nevertheless, but a few general types of cutting. These may be classified as follows: First, the "cabochon" (Fig. 11) type of cutting; second, the old "rose" (Fig. 12) type of cutting; third, the brilliant (Fig. 13); fourth, the step cutting (Fig. 14).

CABOCHONS. Of these the first, or cabochon cutting, is probably the most ancient. The term comes from a French word signifying a bald pate (caboche, from Latin cabo, a head). The usual round cabochon cut closely resembles the top of a head in shape. Cabochon cut stones usually have a flat base, but sometimes a slightly convex base is used, especially in opals and in moonstones, and some stones of very dense color are cut with a concave base to thin them and thus to reduce their color. The contour of the base may be round, or oval, or square, or cushion shape, or heart shape or of any regular form. The top is always smooth and rounding and unfacetted. The relation of the height or thickness to the length or width may be varied to suit the size and shape of the rough piece or to suit one's ideas of symmetry, provided the material be an opaque one, such as turquoise or lapis lazuli. If, however, the material is transparent the best results in the way of the return of light to the front, and hence in the display of the color of the material, are had if the thickness is about one half the spread.



This relation depends upon the refractive index of the material, but as most color stones are of somewhat similar refractive indices, the above proportions are sufficiently accurate for all. The object in view is the securing of total reflection of as much light as possible from the flat polished back of the stone. Cabochon stones are sometimes set over foil or on polished gold to increase the reflection of light.

The path of a ray of light through a cabochon cut stone is closely similar to that through a rose cut diamond [see cut (c) of Fig. 12 for the latter.] Like the rose cut, the cabochon cut does not give much brilliancy as compared to the brilliant cut. Cabochon cut stones, however, have a quiet beauty of color which commends them to people of quiet taste, and even fine rubies, sapphires, and emeralds are increasingly cut cabochon to satisfy the growing demand for fine taste in jewels. The East Indian has all along preferred the cabochon cut for color stones, but possibly his motives have not been unmixed, as the cabochon cut saves a greater proportion of the weight of the rough stone than the more modern types of cutting.

Garnets, more than other stones, have been used in the cabochon cut, and when in that form are usually known as carbuncles (from carbunculus, a glowing coal). Any other fiery red stone might equally well be styled a carbuncle, especially if cabochon cut.



Scientific rubies look very well in the cabochon cut.

Fig. 11 shows in (a) and (b) the front and top of the usual round cabochon. Cut (c) of the same figure gives the front elevation of a cabochon which will light up better than the usual round-topped design. In the round-topped type the central part of the top is so nearly parallel to the back that light can pass right through as through a window pane. If the sloping sides are brought up to a blunt point, as in cut (c) there is very much less loss of light and greater beauty results. The East Indian cabochons are frequently cut in a fashion resembling that suggested.



ROSE CUT STONES. It was natural that the earliest cut stones should have the simple rounded lines of the cabochon cutting, for the first thing that would occur to the primitive worker who aspired to improve upon nature's product, would be the rubbing down of sharp edges and the polishing of the whole surface of the stone. Perhaps the next improvement was the polishing of flat facets upon the rounded top of a cabochon stone. This process gives us the ancient type of cutting known as the rose cut. The drawings (a) and (b) of Fig. 12 show the front elevation and the top and (c) shows the path of a ray of light through a "rose." It will be noted that the general shape resembles that of a round cabochon, but twenty-four triangular facets have been formed upon the top. The well-proportioned rose has a thickness about one half as great as its diameter. Diamonds were formerly cut chiefly in the rose form, especially in the days of the East Indian mines, and even in the early part of the nineteenth century many people preferred finely made roses to the thick, clumsy brilliants of that day. To-day only very small pieces of diamond are cut to "roses." As the material so used frequently results from the cleaving of larger diamonds, the public has come to know these tiny roses as "chips."

The best roses have twenty-four regular facets but small ones frequently receive only twelve, and those are seldom regular in shape and in arrangement. Such roses serve well enough for encrusting watch cases and for similar work, as the flat base of the stone can be set in thin metal without difficulty. About the only gem other than diamond that is now cut to the rose form is garnet. Large numbers of small Bohemian garnets are cut to crude rose form for use in cluster work.



THE BRILLIANT CUT, as its name implies, gives the most complete return of light of any of the forms of cutting. The theory of the brilliant has already been discussed (Lesson XXII. in connection with the cutting of diamond). The shape of the brilliant is too well known to require much description. Most brilliants to-day are cut practically round and the form is that of two truncated cones placed base to base. The upper cone is truncated more than the lower, thus forming the large, flat top facet known as the table of the stone [A, Fig. 13, cut (a)]. The truncating of the lower cone forms the tiny facet known as the culet, which lies opposite to the table and is parallel to the latter [see B, Fig. 13, cut (a)]. The edge of meeting of the two cones is the girdle of the brilliant [CD in cut (a), Fig. 13]. The sloping surface of the upper cone is facetted with thirty-two facets in the full cut brilliant, while the lower cone receives twenty-four.

Small stones sometimes receive fewer facets, to lessen the cost and difficulty of cutting, but by paying sufficient for them full cut brilliants as small as one hundred to the carat may be had. Cut (b) of Fig. 13 shows the proper arrangement of the top facets and cut (c) that of the bottom facets.

When cutting colored stones in the brilliant cut, especially if the material is very costly and its color in need of being darkened or lightened, the lapidary frequently takes liberties with the regular arrangement and proportions depicted in the cuts.

STEP CUTTING. The only remaining type of cutting that is in very general use is the step cut (sometimes known as trap cut). Fig. 14, (a), (b), and (c), shows the front elevation, the top and the back of a square antique step cut stone. The contour may be round or completely square or oblong or of some other shape, just as a brilliant may have any of these contours. The distinctive feature of the step cutting is the several series of parallel-edged quadrangular facets above and below the girdle and the generally rounding character of its cross section. This plump, rounding character permits the saving of weight of the rough material, and by massing the color gives usually a greater depth of color than a brilliant of the same spread would have if cut from similar material. While probably never quite as snappy and brilliant as the regular brilliant cut, a well-proportioned step cut stone can be very brilliant. Many fine diamonds have recently been cut in steps for use in exclusive jewelry.

THE MIXED CUT. The ruby and the emerald are never better in color than when in the full step cut, although rubies are frequently cut in what is known as the mixed cut, consisting of a brilliant cut top and a step cut back. Sapphires and many other colored stones are commonly cut in the mixed cut. Recently it has become common to polish the tops of colored stones with a smooth unfacetted, slightly convex surface, the back being facetted in either the brilliant or the step arrangement. Such stones are said to have a "buffed top." They are less expensive to cut than fully facetted stones and do not have the snappy brilliancy of the latter. They do, however, show off the intrinsic color of the material very well.



LESSON XXV

IMITATIONS OF PRECIOUS STONES

"PASTE" GEMS. Large volumes have been written on paste jewels, especially on antique pastes. Contrary to a prevailing belief, the paste gem is not a recent invention. People frequently say when told that their gems are false, "But it is a very old piece, it must be genuine." The great age of a jewel should rather lead to suspicion that it was not genuine than give confidence that a true gem was assured. The Egyptians and Romans were skillful makers of glass of the sort used in imitating gems and some of the old pastes were very hard or else have become so with age.

Glass of one variety or another makes the most convincing sort of imitation precious stones. The term "paste" as applied to glass imitations is said to come from the Italian pasta meaning dough, and it suggests the softness of the material. Most pastes are mainly lead glass. As we saw in Lesson XVIII., on the chemical composition of the gems, many of them are silicates of metals. Now glasses are also silicates of various metals, but unlike gem minerals the glasses are not crystalline but rather amorphous, that is, without definite geometric form or definite internal arrangement.

The optical properties of the various glasses vary chiefly with their densities, and the denser the material the higher the refractive index and the greater the dispersion. Thus to get the best results in imitation stones they should be made of very heavy glass. The dense flint glass (chiefly a silicate of potassium and lead) which is used for cut glass ware illustrates admirably the optical properties of the heavy glasses. By using even more lead a still denser glass may be had, with even a greater brilliancy.

Unfortunately the addition of lead or other heavy metals (such as thallium) makes the product very soft and also very subject to attack by gases such as are always present in the atmosphere of cities. This softness causes the stones to scratch readily so that when worn they soon lose their polish and with the loss of polish they lose their beauty. The attack of the gases before mentioned darkens the surfaces of the imitation and further dulls it. When fresh and new a well cut piece of colorless paste has a snap and fire that approaches that of diamond. The surface luster is not adamantine, however, and the edges of the facets cannot be polished so sharply as those on a diamond. Moreover the refractive index, while high, is never so high as in a diamond and hence the brilliant cannot be so shaped as to secure the amount of total reflection given by a well-made diamond. Hence, the paste brilliant, while quite satisfying as seen from squarely in front, is weak and dark in the center as seen when tilted to one side. By these differences the trained eye can detect paste imitations of diamond at a glance without recourse to tests of specific gravity, hardness, etc.

Pastes, being amorphous, are singly refracting, as is diamond. This fact helps the appearance of the paste brilliant, for light does not divide within it to become weakened in power. This singleness of refraction, however, betrays the paste imitation when it is colored to resemble ruby, sapphire or emerald, all of which are doubly refracting.

The color is imparted to pastes by the addition, during their manufacture, of various metallic oxides in small proportions. Thus cobalt gives a blue color, copper or chromium green, copper or gold give red (under proper treatment) and manganese gives purple. By experiment the makers of pastes have become very skillful in imitating the color of almost any precious stone. Fine paste emeralds may look better than inferior genuine emeralds.

As pastes are singly refracting and hence lack dichroism, the pleasing variety of color of the true ruby cannot be had in a paste imitation, but the public is not critical enough to notice this lack. The expert would, however, note it and could detect the imitation by that difference as well as by the lack of double refraction. The use of direct sunlight and a white card as already explained in the lesson on double refraction (Lesson III.) will serve to expose the singleness of refraction of paste imitations. Spinels and garnets are about the only true gems (except diamond) that are single refracting. Any other color stone should show double refraction when tested by the sunlight-card method. The file test will also expose any paste imitation as all the very brilliant pastes are fairly soft.

DOUBLETS. To give better wearing quality to paste imitations the doublet was devised. This name is used because the product is in two parts, a lower or back portion of paste and an upper or top portion of some cheap but hard genuine stone. Garnet is probably used for this purpose to a greater extent than any other material, although quartz or colorless topaz will do very well.

The usual arrangement of the parts can be seen in Fig. 15, the garnet covering only a part of the upper surface, namely the table part and a small portion of the sloping surface of the top. In high class doublets the hard mineral covers the paste to the girdle. (See Fig. 16.) The color of the garnet does not interfere seriously with that of the paste.



If a "diamond" doublet is desired the slice of garnet is made nearly as thin as paper and it covers only the table of the brilliant. It is thus practically colorless. A thin slice of red garnet over a green background is not noticeable, as all the red is absorbed in passing through the green material beneath. With a blue base, the red upper layer may give a very slight purple effect. With yellow a slight orange tint results and of course with a red back no perceptible difference would result.



The two materials are cemented together, by means of a transparent waterproof cement. The triplet has already been described in Lesson XII. It is even better than the doublet and more difficult to detect. Both the file test and the sunlight-card test serve to detect doublets, as well as paste imitations, except that in the file test with the fully protected doublet the back of the stone must be tested with the file, as the girdle and top are of hard material.

In the sunlight-card test of a doublet (the refraction of garnet being single like that of glass), single images of the facets will be had on the card when the sunlight is reflected onto it. A reflection of the lower or inner surface of the garnet top can be seen also and this serves to still further identify a doublet or a triplet. The appearance of this reflection is much like that received on the card from the top of the table. It is larger than the reflections of the smaller facets and is but little colored.

TESTS FOR DOUBLETS. A trained eye can also detect a doublet or a triplet by noting the difference in the character of the surface luster of the garnet part and of the glass part. Garnet takes a keener and more resinous luster than glass. By tipping the doublet so that light is reflected to the eye from the sloping top surface, one can see at once where the garnet leaves off and the glass begins. Even through a show window one can tell a doublet in this way although here it is necessary to move oneself, instead of the stone, until a proper position is obtained to get a reflection from the top slope of the doublet.

If the garnet covers the whole top of the imitation then it is not possible to get so direct a comparison, but even here one can look first at the top surface and then at the back and thus compare the luster. It is also well to closely examine with a lens the region of the girdle, to see if any evidence of the joining of two materials can be seen. Frequently the lapidary bevels the edge so as to bring the line of junction between real and false material at the sharp edge of the bevel. Boiling a doublet in alcohol or chloroform will frequently dissolve the cement and separate the parts.

The dichroscope also serves to detect the false character of doublets and paste imitations, as neither shows dichroism. As rubies, emeralds, sapphires, and in fact most colored stones of value, show distinct dichroism, this test is a sure one against these imitations.

Triplets and doublets too may be exposed by dipping them sidewise into oil, thus removing the prismatic refraction almost completely, as the oil has about the same refractive index as the stone. One can then look directly through glass and garnet, or other topping material, separately, and each material then shows its proper color. Thus zones of color appear in a doublet or triplet when under the oil. A real gem would appear almost uniform in color under these conditions.

Round gas bubbles can frequently be found in paste, and hence in the paste part of a doublet. Also, the natural flaws of the real stone are never found in paste, but may be present in the real stone part of a doublet or a triplet. Some imitation emeralds on the market, however, have been made in a way to counterfeit the flaws and faults generally found in this stone.

ALTERED STONES. In addition to the out and out imitations made of paste, and the doublets, there are numerous imitations current in the trade that are made by staining or by otherwise altering the color of some genuine but inexpensive gem material.

For example, large quantities of somewhat porous chalcedony from Brazil are stained and sold in imitation of natural agate or sard or other stones. In many cases the staining is superficial, so that the stone has to be shaped before it is stained, then stained and polished.

Large quantities of slightly crackled quartz are stained to resemble lapis lazuli, and sold, usually with the title "Swiss Lapis." A file test will reveal the character of this imitation, as it is harder than a file, while true lapis is softer. The color too is never of so fine a blue as that of fine lapis. It has a Prussian blue effect.

Turquoises of inferior color are also sometimes stained to improve them. A better product is made artificially.

Opals are sometimes impregnated with organic matter, which is then charred, perhaps with sulphuric acid, thus giving them somewhat the appearance of black opal.

Opals are also imitated by adding oxide of tin to glass, thus imparting a slight milkiness to it. The imitation is then shaped from this glass by molding, and the back of the cabochon is given an irregular surface, which may be set over tinsel to give the effect of "fire."

Pale stones are frequently mounted over foil, or in enameled or stained settings and thus their color is seemingly improved.

Diamonds of poor color are occasionally "painted"; often the back of the brilliant is treated with a violet dyestuff, which even in so small an amount that it is difficult to detect, will neutralize the yellow of the stone and make it appear to be of a fine blue-white color. The "painting" is, of course, not permanent, so that such treatment of a diamond with a view to selling it is fraudulent. The painted stone may be detected by washing it with alcohol, when the dye will be removed and the off-color will become apparent. If the stone is unset one can see with a lens a wavery metallic appearance on the surfaces that have been "painted." This effect is due to the action of the very thin film of dye upon the light that falls upon it.

Besides the staining of genuine materials, they are sometimes altered in color by heat treatment, and this topic will be discussed in the next lesson.



LESSON XXVI

ALTERATION OF THE COLOR OF PRECIOUS STONES

Many gem minerals change color when more or less strongly heated. Extreme heat whitens many colored materials completely.

"PINKED TOPAZ." John Ruskin advises us to "seek out and cast aside all manner of false or dyed or altered stones" but, in spite of his advice, perhaps the most justifiable use of heat treatment is that which alters the color of true topaz from a wine-yellow to a fine pink. It would appear that the wine-yellow is a composite color composed of pink and yellow and that the pink constituent is less easily changed by heat than is the yellow one. If too high a temperature is used both colors disappear and white topaz results. As the latter is abundant in nature and of little value, such a result is very undesirable. Pink topaz, however, is very rare, and until recently, when pink tourmaline from California and Madagascar, and pink beryl (morganite) from Madagascar, became available in quantity, the "pinked" topazes had but few competing gems, and thus commanded a higher price than the natural topazes. Of course, care has to be taken in heating a mineral to raise and lower the temperature slowly, in order to avoid sudden and unequal expansion or contraction, which would crack and ruin the specimen, as the writer learned to his sorrow with the first topaz that he tried to "pink."

SPANISH TOPAZ. Another material that gains a more valuable color by heat treatment is the smoky quartz of Spain, which, on being gently heated, yields the so-called Spanish topaz. Some amethysts are altered to a yellow color by mild heating. Too great a temperature completely decolorizes colored quartz. Some dark quartz yields a nearly garnet red product, after heating.

ZIRCON. Slight increase in temperature causes many of the zircons from Ceylon to change markedly in color. An alcohol flame serves admirably to effect the change, care being taken to warm up the stone very gradually and to cool it slowly. Drafts should be prevented, as they might suddenly cool the stone and crack it. Some zircons become completely whitened by this treatment. At the same time they increase markedly in density and in refractive index and thus become even more snappy and brilliant than when colored. One is tempted to suspect that the "space lattice" of the crystal has had its strata drawn closer together during the heating and left permanently in a closer order of arrangement. Other zircons merely become lighter colored and less attractive. Some of the whitened stones again become more or less colored on exposure to strong light. Ultra-violet light will sometimes restore these to a fine deep color in a short time.

The whitened zircon, when finely cut in the brilliant form, with truly flat facets and sharp edges and with a top angle of about 39 degrees and a back angle of about 44 degrees, so closely resembles a diamond that it will deceive almost anyone on casual inspection. The expert, even, may be deceived, if caught off his guard. The writer has a fine specimen of a little over one carat, with which he has deceived many jewelers and pawnbrokers, and even an importer or two. If it is presented as a stone that closely resembles diamond your expert will say: "Yes, it is pretty good, but it would never fool me." If, however, you catch him off his guard by suggesting, perhaps, "Did you ever see a diamond with a polished girdle?", then he will look at it with interest, remark on its fine color and "make," and never think of challenging its character.

The refractive index of the dense type of zircon is so high (1.92-1.98) that it lights up well over most of the surface of the brilliant when cut, as above indicated, and does not show markedly the weak dark center shown by white sapphire, white topaz, colorless quartz, colorless beryl, and paste, when seen from the side. Moreover, the luster of zircon is nearly adamantine, so the expert does not miss the cold metallic glitter as he would with any other white stone. The color dispersion, too, is so high (86% as great as in diamond) that the zircon has considerable "fire," and thus the casual handler is again deceived. A fine white zircon is really prettier than a poor diamond. It cannot compare, however, with a fine diamond. It would never do to let an expert see your zircon beside even a fair diamond. The zircon would look "sleepy." It is only when no direct comparison is possible, and when the expert is not suspicious, that a zircon can deceive him. Of course, the use of the scientific tests of the earlier lessons will, at once, detect the character of a whitened zircon. The hardness is but 7.5, the refraction so strongly double that the edges of the back facets appear double-lined when viewed through the table with a lens, and the specific gravity is 4.69. Double spots of light appear on the card when the sunlight-card test is applied. Hence, it is easy to detect zircon by any of these tests if there is reason to suspect that it has been substituted for diamond.

CORUNDUM GEMS. Rubies of streaky color are said to be improved by careful heating. Usually ruby undergoes a series of color changes on being heated, but returns through the same series in reverse order on being cooled, and finally resumes its original color. Strong heating will whiten some yellow sapphire. The author thus obtained a white sapphire from a crystal of light yellow material.

It is interesting to note that the corundum gems undergo marked change in color under the influence of radium. A regular series of changes is said to be produced in white sapphire by this means, the final color being yellow. This color may then be removed by heat and the series run through again. It is not stated that a fine red has ever been thus obtained. Perhaps Nature, by her slower methods, using the faint traces of radio-active material in the rocks, reddens the corundum of Burmah at her leisure, and finally arrives at the much sought "pigeon blood" color. It is said that the natives of India have a legend to the effect that the white sapphires of the mines are "ripening rubies," and that one day they will mature. Perhaps they are not far wrong.

DIAMOND. Diamonds of yellowish tint may be improved in color by the use of high-power radium. At present the latter is so rare and costly that there is no evidence of its commercial use for this purpose. Scientists have brought about the change to a light blue as an experiment. It is not yet known whether the change will be permanent. Perhaps here again Nature has anticipated man's discovery and made the fine bluish-violet Brazilian diamonds (which fluoresce to a deep violet under an arc light, and which shine for a few moments in the dark after exposure to light) by associating them for ages with radio-active material. Some of the African stones also have these characteristics.

Aside from the change in the color of diamond that may be brought about by means of radium, the mineral is extremely reluctant to alter its color. Many experimenters besides the author have tried in vain a host of expedients in the hope of finding some way to improve the color of diamond. About the only noticeable alteration that the author has been able to bring about was upon a brown diamond, the color of which was made somewhat lighter and more ashen by heating it in a current of hydrogen gas to a low red heat.



LESSON XXVII

PEARLS

Unlike the gems that have been so far considered, the pearl is not a mineral, but is of organic origin, that is, it is the product of a living organism. There are two principal types of molluscs which yield true pearls in commercial quantities. The best known of the first type is the so-called pearl oyster (Meleagrina margaritifera). The pearl mussel of fresh water streams is of the second type (Unio margaritifera). Other species of molluscs having pearly linings to their shells may produce pearls, but most of the pearls of commerce come from one or the other of the two varieties mentioned.

STRUCTURE OF PEARL. The structure and material of the true pearl must be first understood in order to understand the underlying reasons for the remarkable beauty of this gem. Pearls are composed partly of the mineral substance calcium carbonate (chemically the same as marble) and partly of a tough, horny substance of organic nature called conchiolin. The shell of the pearl-bearing mollusc is also composed of these two substances. Calcium carbonate may crystallize in either of two forms, calcite or aragonite. In marble we have calcite. In the outer portions of the shell of the pearl oyster the calcium carbonate is in the form of calcite, but in the inner nacreous lining and in the pearl itself the mineral is present as aragonite. This is deposited by the mollusc in very thin crystalline layers in the horny layers of conchiolin, so that the lining of the shell is built of approximately parallel layers of mineral and of animal substance. In the normal shell this is all that takes place, but in the case of a mollusc whose interior is invaded by any small source of irritation, such as a borer, or a grain of sand, or other bit of foreign material, a process of alternate deposit of conchiolin and of aragonite goes on upon the invading matter, thus forming a pearl.

The pearl is built in layers like an onion. In shape it may be spherical, or pear-shaped, or button-shaped or of any less regular shape than these. The regular shapes are more highly valued. The spherical shape is of greatest value, other things being equal. Next comes the drop or pear shape, then the button shape, and after these the host of irregular shapes known to the jeweler as "baroques." The river man who gathers mussels calls these odd-shaped pearls "slugs."

Let us now attempt to understand how the beautiful luster and iridescence of the pearl are related to the layer-like structure of the gem. In the first place, it should be understood that both conchiolin and aragonite are translucent, that is, they pass light to a certain extent. The layers being exceedingly thin, light can penetrate a considerable number of them if not otherwise deflected from its course. We thus obtain reflections not merely from the outer surface of a pearl, but from layer after layer within the gem and all these reflections reach the eye in a blended reflection of great beauty. The luster of a pearl is then not purely a surface luster in the usual sense of that term, but it is a luster due to many superposed surfaces. It is so different from other types of luster that we describe it merely as pearly luster even though we find it in some other material, as, for example in certain sapphires, in which it is due to a similar layer-like arrangement of structure.

ORIENT. The fineness of the luster of a pearl, or as is said in the trade, the orient, depends upon the number of layers that take part in the reflection, and this number in turn depends upon the translucency of the material and the thinness of the layers. Very fine pearls usually have very many, very thin layers taking part in the reflection. The degree of translucency, considered apart, is sometimes called the "water" of the pearl.

In addition to their beautiful luster, many pearls display iridescence, and this is due in part, as in the case of the pearly lining of the shell (mother of pearl) to overlapping of successive layers, like the overlapping of shingles on a roof. This gives rise to a lined surface, much like the diffraction grating of the physicist, which is made by ruling a glass plate with thousands of parallel lines to the inch. Such a grating produces wonderful spectra, in which the rainbow colors are widely separated and very vivid. The principal on which this separation of light depends is known as diffraction and cannot be explained here, but a similar effect takes place when light falls on the naturally ruled surface of a pearl and helps produce the play of colors known as iridescence. The thin layers themselves also help to produce the iridescence by interference of light much as in the case of the opal, which has already been discussed.

COLOR. Having explained the cause of the orient and water of pearls, the color must next be considered. Pearls may be had of almost any color, but the majority of fine pearls are white, or nearly so. The fine Oriental pearls frequently have a creamy tint. Among fresh water pearls the creamy tint is less often seen, but fine pink tints occur. Occasionally a black pearl is found and on account of its rarity commands a price nearly as great as that obtainable for a white pearl of similar size and quality.

The value of pearls depends upon several different factors and it is far from an easy matter to estimate the value of a fine specimen. It is much easier to grade and estimate the value of diamonds than to do the same for pearls, and it is only by long and intimate acquaintance with the pearls themselves that one can hope to become expert in deciding values. There are, however, several general factors that govern the value of pearls. Chief among these are: 1, Orient; 2, Color; 3, Texture or Skin; 4, Shape and Size.

FACTORS GOVERNING THE VALUE OF PEARLS. Taking up each of these factors in turn, it may be said of the first that unless a pearl has that fine keen luster known as a fine orient, it is of but limited value. No matter what the size, or how perfect the shape, it is nothing, if dead and lusterless. To have great value the gem must gleam with that soft but lively luster peculiar to fine specimens of pearl. With variations in orient go wide variations in value.

As to color, the choicest pearls are pure white or delicate rose pink or creamy white. Pearls in these shades can be had in numbers and these colors are what might be called regular colors. Fancy-colored pearls have peculiar and irregular values, depending a good deal upon rarity and upon the obtaining of a customer for an odd color. Fine pink and fine black pearls are examples of the type that is meant here.

To be very valuable a pearl must have a smooth even skin, that is, the texture of its surface must be even and regular. It must not have pits or scratches or wrinkles, or little raised spots upon it, or any cracks in it. In connection with this topic of "skin," it may be mentioned that it is sometimes true that a pearl of bad skin or of poor luster may be improved markedly by "peeling" it, as the process is called. As was said above, a pearl is built in layers much like an onion, and it can often be peeled, that is, one or more layers can be removed, thus exposing fresh layers beneath, whose texture and luster may be better than those of the original outside layer.

"PEELING" A PEARL. Possibly an anecdote of an actual case may serve best to explain the method by which "peeling" is sometimes accomplished. The writer was once at Vincennes, Ind., on business, and there became acquainted with a pearl buyer who was stopping at that place to buy fresh water pearls and "slugs" from the rivermen who gather the mussels for the sake of their shells. The latter are made into "pearl" buttons for clothing. It happened that the pearl buyer had accumulated some twenty-eight ounces of slugs and a number of pearls and was leaving on the same train with the author, who shared his seat with him. While we were looking over the slugs together the pearl buyer put his hand in his pocket and drew out a five-dollar bill which he unrolled, exposing a pearl of about six grains, well shaped, but of rather dead luster. Remarking that he had paid but $4 for it and that he had rolled it up in the bill for safe keeping until he got time to peel it, he took out a small penknife, opened one of the blades, put a couple of kid glove finger tips on the thumb and first finger of his left hand and proceeded to peel the pearl on the moving train. Holding his two hands together to steady them, he pressed the edge of his knife blade against the pearl until the harder steel had penetrated straight down through one layer. Then with a flaking, lateral motion he flaked off a part of the outer skin. Bit by bit all of the outer layer was flaked off, and that, too, without appreciably scratching the next layer, so great was the worker's skill. When the pearl was completely peeled it was gently rubbed with three grades of polishing paper, each finer than the previous one, and then the writer was allowed to examine it. The appearance had been much improved, although it was not of extremely fine quality even when peeled. Under a high power magnifier scarcely a trace of the peeling could be seen. The value of the $4 pearl had been raised to at least $100 and not many minutes had been required for the change. A slower and more laborious, but safer, process of "peeling" a pearl, consists in gently rubbing the surface with a very fine, rather soft, abrasive powder until all of the outer skin has been thus worn away.

Of course, in many such cases no better skin than the outer one could be found and disappointment would result from the peeling of such a pearl. It should be added that it will not do to try to peel a part of a pearl in order to remove an excrescence, for then one would inevitably cut across the layers, exposing their edges, and such a surface looks, when polished, much like a pearl button, but not like a pearl.

In this connection may be mentioned the widespread belief on the part of the public that the concretions found in the common edible oyster can be polished by a lapidary, as a rough precious stone can be improved by the latter, and that a fine pearl will result. It is frequently necessary for jewelers to whom such "pearls" are brought, to undeceive the person bringing them and to tell him that only those molluscs that have a beautiful pearly lining to their shells are capable of producing true pearls and that the latter require no assistance from the lapidary.

SHAPE. To return to the topic of factors governing the value of pearls, the shape of the pearl makes a vast difference in the value. Perfectly spherical pearls are most highly valued and closely following come those of drop or pear shape, as this shape lends itself nicely to the making of pendants. Oval or egg-shaped pearls are also good. After these come the button shapes, in which one side is flattened. Pearls of irregular shape are much less highly valued. The irregular-shaped pearls are called baroque pearls in the trade. The rivermen engaged in the fresh water pearl fishery call them slugs. Some of the more regular of these are called "nuggets." Others are termed "spikes" because of their pointed shape, and still others are called "wing" pearls on account of their resemblance to a bird's wing. Most of the baroques are too irregular in shape to have any special name applying to their form.

WEIGHT. After orient, color, skin, and shape have been considered, size or weight finally determines the value. Pearls are sold by an arbitrary unit of weight known as the pearl grain. It is not equal to the grain avoirdupois, but is one fourth of a diamond carat. As the new metric carat is one fifth of a gram and as there are 15.43 avoirdupois grains in a gram, it is seen at once that there are but 3.08 real grains in a carat rather than four. Thus the pearl grain is slightly lighter than the avoirdupois grain.

Since large, fine pearls are exceedingly rare, the value mounts with size much more rapidly than is the case with any other gem; in fact, the value increases as the square of the weight. For example, let us consider two pearls, one of one grain weight, the other of two grains, and both of the same grade as to quality. If the smaller is worth say $2 per grain, then the larger is worth 2 x 2 (the square of the weight) times $2 (the price per grain base, as it is called in the trade), which totals $8. A four-grain pearl of this grade would be worth 4 x 4 x $2 = $32, etc. Thus it is seen that the price increases very rapidly with increase in weight.

PRICE "PER GRAIN BASE." Some of the lower grades of pearls in small sizes are sold by the grain straight, that is, the price per grain is merely multiplied by the weight in grains to get the value, just as the price per carat would be multiplied by the number of carats to get the value of a diamond. This method of figuring the value of pearls is used only for the cheaper grades and small sizes, however, and the method first explained, the calculation per grain base, is the one in universal use for fine gems. Very fine exceptional gems may be sold at a large price for the piece, regardless of the weight.

It is interesting to note in this connection that Tavernier, the French gem merchant of the seventeenth century, tells us that in his day the price of large diamonds was calculated by a method similar to that which we now use for pearls, that is, the weight in carats was squared and the product multiplied by the price per carat. Such a method would give far too high a price for diamonds to-day.

THE HIGH PRICE OF FINE PEARLS. This suggests the thought that pearls of fine quality and great size are the most costly of all gems to-day and yet there seems to be no halting in the demand for them. In fact, America is only just beginning to get interested in pearls and is coming to esteem them as they have long been esteemed in the East and in Europe. Those who have thought that the advance in the prices of diamonds in recent years will soon put them at prohibitive rates should consider the enormous prices that have been obtained and are being obtained for fine pearls.

In order to facilitate the calculating of prices of pearls, tables have been computed and published giving the values of pearls of all sizes at different prices per grain base, and several times these tables have been outgrown, and new ones, running to higher values, have been made. The present tables run to $50 per grain base.

There is much justification for the high prices demanded and paid for large and fine pearls. Such gems are really exceedingly scarce. Those who, as boys, have opened hundreds of river mussels only to find a very few small, badly misshapen "slugs" will realize that it is only one mollusc in a very large number that contains a fine pearl. Moreover, like the bison and the wild pigeon, the pearl-bearing molluscs may be greatly diminished in numbers or even exterminated by the greed of man and his fearfully destructive methods of harvesting nature's productions. In fact, the fisheries have been dwindling in yield for some time, and most of the fine pearls that are marketed are old pearls, already drilled, from the treasuries of Eastern potentates, who have been forced by necessity to accept the high prices offered by the West for part of their treasures. In India, pearls have long been acceptable collateral for loans, and many fine gems have come on the market after failure of the owners to repay such loans.

Having considered the factors bearing on the value of pearls, we will next consider briefly their physical properties. The specific gravity is less definite than with minerals and varies between 2.65 and 2.70. It may be even higher for pink pearls.

PHYSICAL PROPERTIES. In hardness pearls also vary, ranging between 3-1/2 and 4 on Mohs's scale. They are thus very soft and easily worn or scratched by hard usage. A case showing the rather rapid wearing away of pearls recently came to the attention of the writer. A pendant in the shape of a Latin cross had been made of round pearls which had been drilled and strung on two slender gold rods to form the cross. The pearls were free to rotate on the wires. After a period of some twenty or more years of wear the pearls had all become distinctly cylindrical in shape, the rubbing against the garments over which the pendant had been worn having been sufficient to grind away the soft material to that extent. The luster was still good, the pearls having virtually been "peeled" very slowly by abrasion.

CARE OF PEARLS. This example suggests the great care that should be taken by owners of fine pearls to prevent undue rubbing or wear of these valuable but not extremely durable gems. They should be carefully wiped after being worn to remove dust and then put away in a tightly closed case.

Pearls should never be allowed to come in contact with any acid, not even weak acids like lemonade, or punch or vinegar, as, being largely calcium carbonate they are very easily acted upon by acids, and a mere touch with an acid might ruin the surface luster. Being partly organic in nature, pearls are not everlasting, but must eventually decay, as is shown by the powdery condition of very old pearls that have been found with mummies or in ancient ruins. The organic matter has yielded to bacterial attack and decayed, leaving only the powdery mineral matter behind. As heat and moisture are the conditions most conducive to the growth of bacteria, and hence to decay, it would follow that fine pearls should be kept in a dry cool place when not in use.



LESSON XXVIII

CULTURED PEARLS AND IMITATIONS OF PEARLS

CULTURED PEARLS. Like all very valuable gems, pearls have stimulated the ingenuity of man to attempt to make imitations that would pass for genuine. Perhaps the most ingenious, as well as the most natural looking product, is the "cultured pearl." This is really natural pearl on much of its exterior, but artificial within and at the back. In order to bring about this result the Japanese, who originated the present commercial product, but who probably borrowed the original idea from the Chinese, call to their assistance the pearl oyster itself. The oysters are gently opened, small hemispherical discs of mother-of-pearl are introduced between shell and mantle and the oyster replanted. The foreign material is coated by the oyster with true pearly layers as usual, and after several years a sufficiently thick accumulation of pearly layers is thus deposited on the nucleus so that the oyster may be gathered and opened and the cultured pearl removed by sawing it out from the shell to which it has become attached. To the base is then neatly cemented a piece of mother-of-pearl to complete a nearly spherical shape, and the portions of the surface that have not been covered with true pearl are then polished. The product, when set in a proper pearl mounting, is quite convincing and really beautiful.