From this it will clearly be seen how simple a matter it is to isolate the topaz, tourmaline, and all the pyro-electric stones from the non-pyro-electric, for science has not as yet been able to give to spurious stones these same electric properties, however excellent some imitations may be in other respects. Further, almost all minerals lose their electricity rapidly on exposure to atmospheric influences, even to dry air; the diamond retains it somewhat longer than most stones, though the sapphire, topaz, and a few others retain it almost as long again as the diamond, and these electric properties are some of the tests which are used in the examination of precious stones.

Those stones which show electricity on the application of pressure are such as the fluorspar, calcite, and topaz.

With regard to magnetism, the actual cause of this is not yet known with certainty. It is, of course, a self-evident fact that the magnetic iron ore, which is a form of peroxide, commonly known as magnetite, or lodestone, has the power of attracting a magnet when swinging free, or of being attracted by a magnet, to account for which many plausible reasons have been advanced. Perhaps the most reasonable and acceptable of these is that this material contains molecules which have half their substance positively and the other half negatively magnetised.

Substances so composed, of which magnets are an example, may be made the means of magnetising other substances by friction, without they themselves suffering any loss; but it is not all substances that will respond to the magnet. For instance, common iron pyrites, FeS_{2}, is unresponsive, whilst the magnetic pyrites, which varies from 5FeS, Fe_{2}S_{3}, to 6FeS, Fe_{2}S_{3}, and is a sulphide of iron, is responsive both positively and negatively. Bismuth and antimony also are inactive, whilst almost all minerals containing even a small percentage of iron will deflect the magnetic needle, at least under the influence of heat. So that from the lodestone—the most powerfully magnetic mineral known—to those minerals possessing no magnetic action whatever, we have a long, graduated scale, in which many of the precious stones appear, those containing iron in their composition being more or less responsive, as already mentioned, and that either in their normal state, or when heated, and always to an extent depending on the quantity or percentage of iron they contain.

In this case, also, science has not as yet been able to introduce into an artificial stone the requisite quantity of iron to bring it the same analytically as the gem it is supposed to represent, without completely spoiling the colour. So that the behaviour of a stone in the presence of a magnet, to the degree to which it should or should not respond, is one of the important tests of a genuine stone.



CHAPTER XI.

THE CUTTING OF PRECIOUS STONES.

As existing in a state of nature precious stones do not, as a rule, exhibit any of those beautiful and wonderful properties which cause them to be so admired and sought after as to become of great intrinsic value, for their surfaces have become clouded by innumerable fine cuts or abrasions, because of the thousands of years during which they have been under pressure, or tumbled about in rivers, or subjected to the incessant friction caused by surrounding substances. All this occurring above and under ground has given them an appearance altogether different to that which follows cutting and polishing. Further, the shape of the stone becomes altered by the same means, and just as Michael Angelo's figure was already in the marble, as he facetiously said, and all he had to do was to chip off what he did not require till he came to it, so is the same process of cutting and polishing necessary to give to the precious stones their full value, and it is the manner in which these delicate and difficult operations are performed that is now under consideration. Just as experience and skill are essential to the obtaining of a perfect figure from the block of marble, so must the cutting and polishing of a precious stone call for the greatest dexterity of which a workman is capable, experience and skill so great as to be found only in the expert, for in stones of great value even a slight mistake in the shaping and cutting would probably not only be wasteful of the precious material, but would utterly spoil its beauty, causing incalculable loss, and destroying altogether the refrangibility, lustre and colour of the stone, thus rendering it liable to easy fracture: in every sense converting what would have been a rare and magnificent jewel to a comparatively valueless specimen.

One of the chief services rendered by precious stones is that they may be employed as objects of adornment, therefore, the stone must be cut of such a shape as will allow of its being set without falling out of its fastening—not too shallow or thin, to make it unserviceable and liable to fracture, and in the case of a transparent stone, not too deep for the light to penetrate, or much colour and beauty will be lost. Again, very few stones are flawless, and the position in which the flaw or flaws appear will, to a great extent, regulate the shape of the stones, for there are some positions in which a slight flaw would be of small detriment, because they would take little or no reflection, whilst in others, where the reflections go back and forth from facet to facet throughout the stone, a flaw would be magnified times without number, and the value of the stone greatly reduced. It is therefore essential that a flaw should be removed whenever possible, but, when this is not practicable, the expert will cut the stone into such a shape as will bring the defect into the least important part of the finished gem, or probably sacrifice the size and weight of the original stone by cutting it in two or more pieces of such a shape that the cutting and polishing will obliterate the defective portions. Such a method was adopted with the great Cullinan diamond, as described in Chapter IV. From this remarkable diamond a great number of magnificent stones were obtained, the two chief being the largest and heaviest at present known. Some idea of the size of the original stone may be gathered from the fact that the traditional Indian diamond, the "Great Mogul," is said to have weighed 280 carats. This stone, however, is lost, and some experts believe that it was divided, part of it forming the present famous Koh-i-nur; at any rate, all trace of the Great Mogul ceased with the looting of Delhi in 1739. The Koh-i-nur weighs a little over 106 carats; before cutting it weighed a shade over 186; the Cullinan, in the same state, weighed nearly 3254 carats. This massive diamond was cut into about 200 stones, the largest, now placed in "The Royal Sceptre with the Cross," weighing 516-1/2 carats, the second, now placed under the historic ruby in "The Imperial State Crown," weighing 309-3/16ths carats. These two diamonds are now called "The Stars of Africa." Both these stones, but especially the larger, completely overshadow the notorious Koh-i-nur, and notwithstanding the flaw which appeared in the original stone, every one of the resulting pieces, irrespective of weight, is without the slightest blemish and of the finest colour ever known, for the great South African diamond is of a quality never even approached by any existing stone, being ideally perfect.

It requires a somewhat elaborate explanation to make clear the various styles of cut without illustrations. They are usually divided into two groups, with curved, and with flat or plane surfaces. Of the first, the curved surfaces, opaque and translucent stones, such as the moonstone, cat's-eye, etc., are mostly cut en cabochon, that is, dome-shaped or semi-circular at the top, flat on the underside, and when the garnet is so cut it is called a carbuncle. In strongly coloured stones, while the upper surface is semi-circular like the cabochon, the under surface is more or less deeply concave, sometimes following the curve of the upper surface, the thickness of the stone being in that case almost parallel throughout. This is called the "hollow" cabochon. Other stones are cut so that the upper surface is dome-shaped like the last two, but the lower is more or less convex, though not so deep as to make the stone spherical. This is called the "double" cabochon.

A further variety of cutting is known as the goutte de suif, or the "tallow-drop," which takes the form of a somewhat flattened or long-focus double-convex lens. The more complicated varieties of cut are those appearing in the second group, or those with plane surfaces. A very old form is the "rose" or "rosette"; in this the extreme upper centre, called the "crown," or "star," is usually composed of six triangles, the apexes of which are elevated and joined together, forming one point in the centre. From their bases descend a further series of triangles, the bases and apexes of which are formed by the bases and lower angles of the upper series. This lower belt is called the "teeth," under which the surface or base of the stone is usually flat, but sometimes partakes of a similar shape to the upper surface, though somewhat modified in form.

Another variety is called the "table cut," and is used for coloured stones. It has a flat top or "table" of a square or other shape, the edges of which slope outwards and form the "bezils" or that extended portion by which the stone is held in its setting. It will thus be seen that the outside of the stone is of the same shape as that of the "table," but larger, so that from every portion of the "table" the surface extends downwards, sloping outwards to the extreme size of the stone, the underside sloping downwards and inwards to a small and flat base, the whole, in section, being not unlike the section of a "pegtop."

A modification of this is known as the "step" cut, sometimes also called the "trap." Briefly, the difference between this and the last is that whereas the table has usually one bevel on the upper and lower surfaces, the trap has one or more steps in the sloping parts, hence its name.

The most common of all, and usually applied only to the diamond, is the "brilliant" cut. This is somewhat complicated, and requires detailed description. In section, the shape is substantially that of a pegtop with a flat "table" top and a small flat base. The widest portion is that on which the claws, or other form of setting, hold it securely in position. This portion is called the "girdle," and if we take this as a defining line, that portion which appears above the setting of this girdle, is called the "crown"; the portion below the girdle is called the "culasse," or less commonly the "pavilion." Commencing with the girdle upwards, we have eight "cross facets" in four pairs, a pair on each side; each pair having their apexes together, meeting on the four extremities of two lines drawn laterally at right angles through the stone. It will, therefore, be seen that one side of each triangle coincides with the girdle, and as their bases do not meet, these spaces are occupied by eight small triangles, called "skill facets," each of which has, as its base, the girdle, and the outer of its sides coincides with the base of the adjoining "cross facet." The two inner sides of each pair of skill facets form the half of a diamond or lozenge-shaped facet, called a "quoin," of which there are four. The inner or upper half of each of these four quoins forms the bases of two triangles, one at each side, making eight in all, which are called "star facets," and the inner lines of these eight star facets form the boundary of the top of the stone, called the "table." The inner lines also of the star facets immediately below the table and those of the cross facets immediately above the girdle form four "templets," or "bezils." We thus have above the girdle, thirty-three facets: 8 cross, 8 skill, 4 quoin, 8 star, 1 table, and 4 templets.

Reversing the stone and again commencing at the girdle, we have eight "skill facets," sometimes called the lower skill facets, the bases of which are on the girdle, their outer sides forming the bases of eight cross facets, the apexes of which meet on the extremities of the horizontal line, as in those above the girdle. If the basal lines of these cross facets, where they join the sides of the skill facets, are extended to the peak, or narrow end of the stone, these lines, together with the sides of the cross facets, will form four five-sided facets, called the "pavilions"; the spaces between these four pavilions have their ends nearest the girdle formed by the inner sides of the skill facets, and of these spaces, there will, of course, be four, which also are five-sided figures, and are called "quoins," so that there are eight five-sided facets—four large and four narrow—their bases forming a square, with a small portion of each corner cut away; the bases of the broader pavilions form the four sides, whilst the bases of the four narrower quoins cut off the corners of the square, and this flat portion, bounded by the eight bases, is called the "culet," but more commonly "collet." So that below the girdle, we find twenty-five facets: 8 cross, 8 skill, 4 pavilion, 4 quoin, and 1 collet.

These, with the 33 of the crown, make 58, which is the usual number of facets in a brilliant, though this varies with the character, quality, and size of the diamond. For instance, though this number is considered the best for normal stones, specially large ones often have more, otherwise there is danger of their appearing dull, and it requires a vast amount of skill and experience to decide upon the particular number and size of the facets that will best display the fire and brilliance of a large stone, for it is obvious that if, after months of cutting and polishing, it is found that a greater or smaller number of facets ought to have been allowed, the error cannot be retrieved without considerable loss, and probable ruin to the stone. In the case of the Cullinan diamonds, the two largest of which are called the Stars of Africa, 74 facets were cut in the largest portion, while in the next largest the experts decided to make 66, and, as already pointed out, these stones are, up to the present time, the most magnificent in fire, beauty and purity ever discovered.

The positions and angles of the facets, as well as the number, are of supreme importance, and diamond cutters—even though they have rules regulating these matters, according to the weight and size of the stone—must exercise the greatest care and exactitude, for their decision once made is practically unalterable.



CHAPTER XII.

IMITATIONS, AND SOME OF THE TESTS, OF PRECIOUS STONES.

We now arrive at the point where it is necessary to discuss the manufacture and re-formation of precious stones, and also to consider a few of the tests which may be applied to all stones. These are given here in order to save needless repetition; the tests which are specially applicable to individual stones will more properly be found under the description of the stone referred to, so that the present chapter will be devoted chiefly to generalities.

With regard to diamonds, the manufacture of these has not as yet been very successful. As will be seen on reference to Chapter II., on "the Origin of Precious Stones," it is generally admitted that these beautiful and valuable minerals are caused by chemically-charged water and occasionally, though not always, high temperature, but invariably beautified and brought to the condition in which they are obtained by the action of weight and pressure, extending unbroken through perhaps ages of time.

In these circumstances, science, though able to give chemical properties and pressure, cannot, of course, maintain these continuously for "ages," therefore the chemist must manufacture the jewels in such manner that he may soon see the results of his labours, and though real diamonds may be made, and with comparative ease, from boron in the amorphous or pure state along with aluminium, fused in a crucible at a high temperature, these diamonds are but microscopic, nor can a number of them be fused, or in any other way converted into a large single stone, so that imitation stones, to be of any service must be made of a good clear glass. The glass for this purpose is usually composed of 53.70 per cent. of red lead, 38.48 per cent. of pure quartz in fine powder, preferably water-ground, and 7.82 per cent. of carbonate of potash, the whole coloured when necessary with metallic oxides of a similar nature to the constituents of the natural stones imitated. But for colourless diamonds, the glass requires no such addition to tint it. From the formula given is made the material known as "strass," or "paste," and stones made of it are mostly exhibited under and amongst brilliant artificial lights. The mere fact that they are sold cheaply is prima facie proof that the stones are glass, for it is evident that a diamond, the commercial value of which might be L50 or more, cannot be purchased for a few shillings and be genuine. So long as this is understood and the stone is sold for the few shillings, no harm is done; but to offer it as a genuine stone and at the price of a genuine stone, would amount to fraud, and be punishable accordingly. Some of these "paste," or "white stones," as they are called in the trade, are cut and polished exactly like a diamond, and with such success as occasionally to deceive all but experts. Such imitations are costly, though, of course, not approaching the value of the real stones; it being no uncommon thing for valuable jewels to be duplicated in paste, whilst the originals are kept in the strong room of a bank or safe-deposit.

In all cases, however, a hard file will abrade the surface of the false stone. In chapter VII. we found that quartz is in the seventh degree of hardness, and an ordinary file is but a shade harder than this, so that almost all stones higher than No. 7 are unaffected by a file unless it is used roughly, so as to break a sharp edge. In order to prepare artificial diamonds and other stones for the file and various tests, they are often what is called "converted" into "doublets" or "triplets." These are made as follows: the body of the glass is of paste, and on the "table" (see last chapter), and perhaps on the broader facets, there will be placed a very thin slab of the real stone, attached by cement. In the case of the diamond, the body is clear, but in the coloured imitations the paste portion is made somewhat lighter in shade than the real stone would be, the portion below the girdle being coloured chemically, or mounted in a coloured backing. Such a stone will, of course, stand most tests, for the parts usually tested are genuine.

A stone of this nature is called a "doublet," and it is evident that when it is tested on the underside, it will prove too soft, therefore the "triplet" has been introduced. This is exactly on the lines of the doublet, except that the collet and perhaps the pavilions are covered also, so that the girdle, which is generally encased by the mounting, is the only surface-portion of paste. In other cases the whole of the crown is genuine, whilst often both the upper and lower portions are solid and genuine, the saving being effected by using a paste centre at the girdle, covered by the mounting. Such a stone as this last mentioned is often difficult to detect without using severe tests and desperate means, e.g.:—(a) by its crystalline structure (see Chapter III.); (b) by the cleavage planes (see Chapter IV.); (c) by the polariscope (see Chapter V.); (d) by the dichroscope (see Chapter VI.); (e) by specific gravity (see Chapter VIII.); (f) cutting off the mounting, and examining the girdle; (g) soaking the stone for a minute or so in a mixture said to have been originally discovered by M. D. Rothschild, and composed of hydrofluoric acid and ammonia; this will not answer for all stones, but is safe to use for the diamond and a few others. Should the jewel be glass, it will be etched, if not completely destroyed, but if genuine, no change will be apparent; (h) soaking the diamond for a few minutes in warm or cold water, in alcohol, in chloroform, or in all these in turn, when, if a doublet, or triplet, it will tumble to pieces where joined together by the cement, which will have been dissolved. It is, however, seldom necessary to test so far, for an examination under the microscope, even with low power, is usually sufficient to detect in the glass the air-bubbles which are almost inseparable from glass-mixtures, though they do not detract from the physical properties of the glass. The higher powers of the same instrument will almost always define the junction and the layer or layers of cement, no matter how delicate a film may have been used. Any one of these tests is sufficient to isolate a false stone.

Some of the softer genuine stones may be fused together with splinters, dust, and cuttings of the same stones, and of this product is formed a larger stone, which, though manufactured, is essentially perfectly real, possessing exactly the same properties as a naturally formed stone. Many such stones are obtained as large as an ordinary pin's head, and are much used commercially for cluster-work in rings, brooches, for watch-jewels, scarf-pins, and the like, and are capable of being cut and polished exactly like an original stone. This is a means of using up to great advantage the lapidary's dust, and though these products are real stones, perhaps a little more enriched in colour chemically, they are much cheaper than a natural stone of the same size and weight.

Some spurious stones have their colour improved by heat, by being tinged on the outside, by being tinted throughout with a fixed colour and placed in a clear setting; others, again, have a setting of a different hue, so that the reflection of this shall give additional colour and fire to the stone. For instance, glass diamonds are often set with the whole of the portion below the girdle hidden, this part of the stone being silvered like a mirror. Others are set open, being held at the girdle only, the portion covered by the setting being silvered. Other glass imitations, such as the opal, have a tolerably good representation of the "fiery" opal given to them by the admixture, in the glass, of a little oxide of tin, which makes it somewhat opalescent, and in the setting is placed a backing of red, gold, copper, or fiery-coloured tinsel, whilst the glass itself, at the back, is painted very thinly with a paint composed of well washed and dried fish-scales, reduced to an impalpable powder, mixed with a little pure, refined mastic, or other colourless varnish. This gives a good imitation of phosphorescence, as well as a slight pearliness, whilst the tinsel, seen through the paint and the curious milkiness of the glass, gives good "fire."

A knowledge of the colours natural to precious stones and to jewels generally is of great service in their rough classification for testing, even though some stones are found in a variety of colours. An alphabetical list of the most useful is here appended, together with their average specific gravities and hardness. (See also Chapter VII. on "Hardness," and Chapter VIII. on "Specific Gravity.")

WHITE OR COLOURLESS STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.)

Beryl 7-3/4 2.709-2.81 Corundum 9 3.90-4.16 Diamond 10 3.502-3.564 Jade 7 3.300-3.381 Opal 5-1/2-6-1/2 2.160-2.283 Phenakite 7-3/4 2.965 Quartz 7 2.670 Rock-crystal 7 2.521-2.795 Sapphire 9 4.049-4.060 Spinel 8 3.614-3.654 Topaz 8 3.500-3.520 Tourmaline 7-1/4 3.029 Zircon 7-1/2 4.700-4.880

YELLOW STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Amber 2-1/2 1.000 Beryl 7-3/4 2.709-2.810 Chrysoberyl 8-1/2 3.689-3.752 Chrysolite 6-7 3.316-3.528 Corundum (the yellow variety known as "Oriental Topaz" [not "Topaz"], see below) 9 3.90-4.16 Diamond 10 3.502-3.564 Garnets (various) 6-1/2-7-1/2 3.4-4.5 Hyacinth (a form of Zircon) 7-1/2 4.7-4.88 Quartz (Citrine) 7 2.658 Sapphire 9 4.049-4.060 Spinel 8 3.614-3.654 Topaz (for "Oriental Topaz," see above) 8 3.500-3.520 Tourmaline 7-1/4 3.210

BROWN AND FLAME-COLOURED STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Andalusite 7-1/2 3.204 Diamond 10 3.502-3.564 Garnets (various) 6-1/2-7-1/2 3.40-4.50 Hyacinth (a form of Zircon), see below 7-1/2 4.70-4.88 Quartz (smoke coloured) 7 2.670 Tourmaline 7-1/4 3.100 Zircon (Hyacinth) 7-1/2 4.70-4.88

RED AND ROSE-COLOURED STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Carnelian (a variety of Chalcedony) 6-1/2 2.598-2.610 Diamond 10 3.502-3.564 Deep Red Garnet 7-1/4 3.40-4.50 Jasper 7 2.668 Opal (the "Fire Opal") 5-1/2-6-1/2 2.21 (average) Ruby 9 4.073-4.080 Rhodonite 5-1/2-6-1/2 3.413-3.617 Sapphire 9 4.049-4.060 Spinel Ruby 8 3.614-3.654 Topaz 8 3.500-3.520 Tourmaline 7-1/4 3.024 Zircon 7-1/2 4.70-4.88

PINK STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Beryl 7-3/4 2.709-2.810 Diamond 10 3.502-3.564 Ruby 9 4.073-4.080 Spinel 8 3.614-3.654 Topaz ("burnt" or "pinked"), see Chapter XIV., page 92 8 3.500-3.520 Tourmaline 7-1/4 3.024

BLUE STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Beryl 7-3/4 2.709-2.810 Diamond 10 3.502-3.564 Dichorite (Water Sapphire) 7-7-1/2 4.049-4.060 Disthene (Kyanite) 5-7 3.609-3.688 Iolite (Cordierite) 7-1/4 2.641 Lapis lazuli 5-1/2 2.461 Sapphire 9 4.049-4.060 Topaz 8 3.500-3.520 Tourmaline 7-1/4 3.160 Turquoise 6 2.800

GREEN STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Aquamarine 7-3/4 2.701-2.800 Chrysoberyl 8-1/2 3.689-3.752 Chrysolite 6-7 3.316-3.528 Chrysoprase (Quartz) 7 2.670 Diamond 10 3.502-3.564 Dioptase 5 3.289 Emerald and Oriental Emerald 7-3/4 2.690 Euclase 7-1/2 3.090 Garnet (see also Red Garnet) 6-1/2-7-1/2 3.400-4.500 Heliotrope (Chalcedony) 6-1/2 2.598-2.610 Hiddenite (a variety of Spodumene) 6-1/2-7 3.130-3.200 Jade 7 3.300-3.381 Jadeite 7 3.299 Malachite 3-1/2 3.710-3.996 Peridot (a variety of Chrysolite) 6-7 3.316-3.528 Plasma (a variety of Chalcedony) 6-1/2 2.598-2.610 Quartz 7 2.670 Sapphire 9 4.049-4.060 Topaz 8 3.500-3.520 Tourmaline 7-1/4 3.148

VIOLET STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Amethyst 7 2.661 Diamond 10 3.502-3.564 Quartz (Amethyst) 7 2.670 Sapphire 9 4.049-4.060 Spinel 8 3.614-3.654 Tourmaline 7-1/4 3.160

CHATOYANT STONES.

These stones are easily recognisable by their play of colour. (See Chapter XIV.)

BLACK STONES.

Hardness. Specific Gravity. (See Chapter VII.) (See Chapter VIII.) Diamond 10 3.502-3.564 Garnet 6-1/2-7-1/2 3.400-4.500 Jet 3-1/2 1.348 Onyx (a variety of Chalcedony) 6-1/2 2.598-2.610 Quartz 7 2.670 Tourmaline (not unlike Black Resin in appearance) 7-1/4 3.024-3.300



CHAPTER XIII.

VARIOUS PRECIOUS STONES.

The Diamond.

To recapitulate certain of the facts respecting the diamond.—This wonderful gem has the distinction amongst precious stones of being unique; though many are composed of two, three, or but a small number of elements, the diamond is the only stone known consisting of one element, and absolutely nothing else—pure crystallised carbon. Its hardness is proverbial; not only is it untouched by the action of a hard file, but it occasionally refuses to split when struck with finely tempered steel, which it often causes to break. Such was the case with the South African diamond, for when the knife that was to break it was struck smartly with a steel bar, the first blow broke the blade without affecting the diamond, yet a piece of bort, or diamond dust, splinters, or defective diamonds (all these being called bort), may readily be pulverised in a hard steel mortar with a hard steel pestle.

The diamond is the hardest stone known; it is also the only stone known which is really combustible. It is of true adamantine lustre, classed by experts as midway between the truly metallic and the purely resinous. In refractive power and dispersion of the coloured rays of light, called its fire, it stands pre-eminent. It possesses a considerable variety of colour; that regarded as the most perfect and rare is the blue-white colour. Most commonly, however, the colours are clear, with steely-blue casts, pale and neutral-colour yellow, whilst amongst the most expensive and rare are those of green, pale pink, red, and any other variety with strong and decided colour. Although these stones are sold by the carat, there can be no hard and fast rule laid down as to the value of a carat, for this depends on the size, quality, and the purity of the stone. The larger the stone the greater the value per carat, and prices have been known to range from 25l. per carat for a small stone to 500l. per carat for a large one, whereas the exceptionally large stones possess a value almost beyond estimation.

It often happens that some stones—particularly those from South Africa and Brazil—are tinted when uncut, probably by reason of the action upon them of their matrix, especially if ironstone, or with rolling for ages amongst ironstone in river-beds, which gives them a slight metallic appearance; in each case the cause is suggested by the fact that these tinted stones are usually found in such places, and that the tinting is very thin and on the surface only, so that the cutting and shaping of the stone gets below it to the perfectly clear diamond.

From Pliny and other historians we gather that at various periods considerable superstition has existed with regard to diamonds, such as that if one is powdered it becomes poisonous to a remarkable degree; that gifts of diamonds between lovers—married and unmarried—produce and seal affection; hence the popularity of diamonds in betrothal rings. Pretty as is this conceit, there is no doubt about the fact that the gift of diamonds to the object of one's affections does usually produce a feeling of pleasure to both parties, from which it would appear that there is some ground for the belief.

Corundum.

This mineral is a species of crystal, or crystalline alumina—an almost pure anhydrous alumina, Al{2}O{3}—in many varieties, both of shape and colour. The chief stone is the ruby, considered, when large, to be of even more importance and value than the diamond. There are many other red stones in this group; sapphires, also, are a species of corundum, both the blue and the colourless varieties, as are also the aquamarine, the emerald, the amethyst, the topaz, and others, all of widely differing colour, as well as the star-shaped, or "aster" ruby, called the "ruby" cat's-eye. All these vary more in colour than in their chemical properties. Still another variety, greyish-black and generally associated with haematite iron ore, is called emery, and, when ground in different degrees of fineness, is so well known by its general use as a polishing medium as to need no description. It should, however, be mentioned that amongst the more coarsely ground emery it is no uncommon thing to find minute sapphires, taking sapphires in their broad, commercial meaning, as signifying any variety of corundum, except the red and the emery. The surfaces of crystals of corundum are often clouded or dull, whilst its classification of lustre is vitreous. It is double refracting and has no cleavage. It is found in China, India, Burma, Ceylon, South Africa, America, and in many other places, having a wide distribution.

The Ruby.

In the dichroscope the ruby shows two images, one square of a violet red, the second square being a truer and a paler red. It may be subjected to strong heat, when it changes its colour to a sooty or dirty slate, this varying with the locality in which the stone is found, and the manner in which the heat is applied. But as it cools it becomes paler and greener, till it slowly enrichens; the green first becomes broken, then warmer, redder, and finally assumes its original beautiful blood red. This method of heating is sometimes used as a test, but it is a test which often means the complete ruin of a stone which is not genuine. Another characteristic which, in the eyes of the expert, invariably isolates a real from an artificial ruby is its curious mild brilliance, which as yet has not been reproduced by any scientific method in paste or any other material, but perhaps the safest test of all is the crystalline structure, which identical structure appears in no other stone, though it is possible, by heating alumina coloured with oxide of iron and perhaps also a trace of oxide of chromium to a very high temperature for a considerable time, and then cooling very slowly, to obtain a ruby which is nearly the same in its structure as the real gem; its specific gravity and hardness may perhaps be to standard, and when properly cut, its brilliance would deceive all but an expert. And as in some real rubies there are found slight hollows corresponding or analogous to the bubbles found in melted glass, it becomes a matter of great difficulty to distinguish the real from the imitation by such tests as hardness, specific gravity, dichroism, and the like, so that in such a case, short of risking the ruin of the stone, ordinary persons are unable to apply any convincing tests. Therefore, only the expert can decide, by his appreciation of the delicate shade of difference in the light of a true ruby and that of an excellent imitation, and by the distribution of the colour, which—however experienced the chemist may be, or with what care the colouring matter may have been incorporated in the mass—has been found impossible of distribution throughout the body of an artificial stone so perfectly and in the same manner and direction as nature herself distributes it in the genuine. This alone, even in the closest imitations, is clear to the eye of the expert, though not to the untrained eye, unless the stone is palpably spurious. To one who is accustomed to the examination of precious stones, however perfect the imitation, it is but necessary to place it beside or amongst one or more real ones for the false to be almost instantly identified, and that with certainty.

The Sapphire.

The Sapphire is not so easy to imitate, as its hardness exceeds that of the ruby, and imitations containing its known constituents, or of glass, are invariably softer than the natural stone. As before remarked, almost any form of corundum other than red is, broadly, called sapphire, but giving them their strictly correct designations, we have the olivine corundum, called "chrysolite" (oriental), which is harder than the ordinary or "noble" chrysolite, sometimes called the "peridot." The various yellow varieties of corundum take the name of the "oriental topaz," which, like most, if not all, the corundum varieties, is harder than the gem which bears the same name, minus the prefix "oriental." Then we have the "amethyst" sapphire, which varies from a red to a blue purple, being richer in colour than the ordinary amethyst, which is a form of violet-coloured quartz, but the corundum variety, which, like its companions, is called the "oriental" amethyst, is both rarer and more precious. A very rare and extremely beautiful green variety is called the oriental emerald. The oriental jacinth, or hyacinth, is a brown-red corundum, which is more stable than the ordinary hyacinth, this latter being a form of zircon; it changes colour on exposure to light, which colour is not restored by subsequent retention in darkness.

The blue sapphire is of all shades of blue, from cornflower blue to the very palest tints of this colour, all the gradations from light to dark purple blues, and, in fact, so many shades of tone and colour that they become almost as numerous as the stones. These stones are usually found in similar situations to those which produce the ruby, and often along with them. The lighter colours are usually called females, or feminine stones, whilst the darker ones are called masculine stones. Some of these dark ones are so deep as to be almost black, when they are called "ink" sapphires, and if inclining to blue, "indigo" sapphires, in contradistinction to which the palest of the stones are called "water" sapphires. The colouring matter is not always even, but is often spread over the substance of the stone in scabs or "splotches," which rather favours imitation, and, where this unevenness occurs, it may be necessary to cut or divide the stone, or so to arrange the form of it that the finished stone shall be equally blue throughout.

In some cases, however, the sapphire may owe its beauty to the presence of two, three or more colours in separate strata appearing in one stone; such as a portion being a green-blue, another a cornflower blue, another perfectly colourless, another a pale sky blue, another yellow, each perfectly distinct, the stone being cut so as to show each colour in its full perfection.

This stone, the sapphire, is hardness No. 9 (see "Hardness" table), and therefore ranks next to the diamond, which makes it a matter of great difficulty to obtain an imitation which is of the same specific gravity and of the same degree of hardness, though this has been done. Such stones are purchasable, but though sold as imitations at comparatively low price, and the buyer may consider them just as good as the real gem, to the experienced eye they are readily detectable.

By heating a sapphire its blue colour slowly fades, to complete transparency in many cases, or at any rate to so pale a tint as to pass for a transparent stone. Valuable as is the sapphire, the diamond is more so, and it follows that if one of these clear or "cleared" sapphires is cut in the "rose" or "brilliant" form—which forms are reserved almost exclusively for the diamond—such a stone would pass very well as a diamond, and many so cut are sold by unscrupulous people as the more valuable stone, which fraud an expert would, of course, detect.

Sapphires are mentioned by Pliny, and figure largely in the ancient history of China, Egypt, Rome, etc. The Greeks dedicated the sapphire specially to Jupiter, and many of the stones were cut to represent the god; it also figured as one of the chief stones worn by the Jewish High Priest on the breast-plate. Some stones have curious rays of variegated colour, due to their crystalline formation, taking the shape of a star; these are called "asteriated," or "cat's eye" sapphires. Others have curious flashes of light, technically called a "play" of light (as described in Chapter VI. on "Colour"), together with a curious blue opalescence; these are the "girasol." Another interesting variety of this blue sapphire is one known as "chatoyant"; this has a rapidly changing lustre, which seems to undulate between a green-yellow and a luminous blue, with a phosphorescent glow, or fire, something like that seen in the eyes of a cat in the dark, or the steady, burning glow observed when the cat is fascinating a bird—hence its name. This is not the same variety as the "asteriated," or "cat's eye" or "lynx eye" mentioned above.



CHAPTER XIV.

VARIOUS PRECIOUS STONES—continued.

The Chrysoberyl.

There are certain stones and other minerals which, owing to their possession of numerous microscopically fine cavities, of a globular or tubular shape, have the appearance of "rays" or "stars," and these are called "asteriated." Several of such stones have been discussed already in the last chapter, and in addition to these star-like rays, some of the stones have, running through their substance, one or more streaks, perhaps of asbestos or calcite, some being perfectly clear, whilst others are opalescent. When these streaks pass across the star-like radiations they give the stone the appearance of an eye, the rays forming the iris, the clear, opalescent, or black streak closely resembling the slit in a cat's eye, and when these stones are cut en cabochon, that is, dome-shaped (see Chapter XI. on "Cutting"), there is nothing to deflect the light beams back and forth from facet to facet, as in a diamond, so that the light, acting directly on these radiations or masses of globular cavities and on the streak, causes the former to glow like living fire, and the streak appears to vibrate, palpitate, expand, and contract, exactly like the slit in the eye of a cat.

There are a considerable number of superstitions in connection with these cat's-eye stones, many people regarding them as mascots, or with disfavour, according to their colour. When possessing the favourite hue or "fire" of the wearer, such as the fire of the opal for those born in October, of the ruby for those born in July, etc., these stones are considered to bring nothing but good luck; to ward off accident, danger, and sudden death; to be a charm against being bitten by animals, and to be a protection from poison, the "evil eye," etc. They figured largely, along with other valuable jewels, in the worship of the ancient Egyptians, and have been found in some of the tombs in Egypt. They also appeared on the "systrum," which was a sacred instrument used by the ancient Egyptians in the performance of their religious rites, particularly in their sacrifices to the goddess Isis. This, therefore, may be considered one of their sacred stones, whilst there is some analogy between the cat's-eye stones and the sacred cat of the Egyptians which recurs so often in their hieroglyphics; it is well known that our domestic cat is not descended from the wild cat, but from the celebrated cat of Egypt, where history records its being "domesticated" at least thirteen centuries B.C. From there it was taken throughout Europe, where it appeared at least a century B.C., and was kept as a pet in the homes of the wealthy, though certain writers, speaking of the "mouse-hunters" of the old Romans and Greeks, state that these creatures were not the Egyptian cat, but a carniverous, long-bodied animal, after the shape of a weasel, called "marten," of the species the "beech" or "common" marten (mustela foina), found also in Britain to-day. It is also interesting to note that the various superstitions existing with regard to the different varieties and colours of cats also exist in an identical manner with the corresponding colours of the minerals known as "cat's eye."

Several varieties of cat's-eye have already been described. Another important variety is that of the chrysoberyl called "cymophane." This is composed of glucina, which is glucinum oxide, or beryllia, BeO, of which there is 19.8 per cent., and alumina, or aluminium oxide, Al{2}O{3}, of which there is 80.2 per cent. It has, therefore, the chemical formula, BeO,Al{2}O{3}. This stone shows positive electricity when rubbed, and, unlike the sapphires described in the last chapter, which lose their colour when heated, this variety of chrysoberyl shows no change in colour, and any electricity given to it, either by friction or heat, is retained for a long time. When heated in the blowpipe alone it remains unaltered, that is, it is not fusible, and even with microcosmic salt it requires a considerably long and fierce heat before it yields and fuses, and acids do not act upon it. It crystallises in the 4th (rhombic) system, and its lustre is vitreous.

The cymophane shows a number of varieties, quite as many as the chrysoberyl, of which it is itself a variety, and these go through the gamut of greens, from a pale white green to the stronger green of asparagus, and through both the grey and yellow greens to dark. It is found in Ceylon, Moravia, the Ural Mountains, Brazil, North America, and elsewhere. The cat's-eye of this is very similar to the quartz cat's-eye, but a comparison will make the difference so clear that they could never be mistaken, apart from the fact that the quartz has a specific gravity considerably lower than the chrysoberyl cat's-eye, which latter is the true cat's-eye, and the one usually understood when allusion is made to the stone without any distinguishing prefix, such as the ruby, sapphire, quartz, etc., cat's eye. It should, however, be mentioned that this stone is referred to when the names Ceylonese and Oriental cat's-eye are given, which names are used in the trade as well as the simple appellation, "cat's eye." One peculiarity of some of these stones is that the "fire" or "glow" is usually altered in colour by the colour of the light under which it is seen, the change of colour being generally the complementary. Thus, a stone which in one light shows red, in another will be green; the "eye" showing blue in one light will become orange in another; whilst the yellow of another stone may show a decided purple or amethyst in a different light.

A good test for this, and indeed most precious stones, is that they conduct heat more quickly than does glass, and with such rapidity that on breathing upon a stone the warmth is conducted instantly, so that, though the stone is dimmed the dimness vanishes at once, whereas with glass the film of moisture fades but slowly in comparison.

The Topaz.

The name topaz is derived from the Greek topazos, which is the name of a small island situated in the Gulf of Arabia, from whence the Romans obtained a mineral which they called topazos and topazion, which mineral to-day is termed chrysolite. The mineral topaz is found in Cornwall and in the British Isles generally; also in Siberia, India, South America and many other localities, some of the finest stones coming from Saxony, Bohemia, and Brazil, especially the last-named. The cleavage is perfect and parallel to the basal plane. It crystallises in the 4th (rhombic) system; in lustre it is vitreous; it is transparent, or ranging from that to translucent; the streak is white or colourless. Its colour varies very much—some stones are straw-colour, some are grey, white, blue, green, and orange. A very favourite colour is the pink, but in most cases this colour is not natural to the stone, but is the result of "burning," or "pinking" as the process is called technically, which process is to raise the temperature of a yellow stone till the yellow tint turns to a pink of the colour desired. The topaz is harder than quartz, as will be seen on reference to the "Hardness" table, and is composed of a silicate of aluminium, fluorine taking the place of some of the oxygen. Its composition averages 16.25 per cent. of silica, 55.75 per cent. of alumina, or oxide of aluminium, and fluoride of silicium, 28 per cent. Its formula is [Al(F,OH)]{2} SiO{4}, or (AlF){2}SiO{4}. From this it will be understood that the fluorine will be evolved when the stone is fused. It is, however, very difficult to fuse, and alone it is infusible under the blowpipe, but with microcosmic salt it fuses and evolves fluorine, and the glass of the tube in the open end of which the stone is fixed is bitten with the gas.

Such experiments with the topaz are highly interesting, and if we take a little of the powdered stone and mix with it a small portion of the microcosmic salt, we may apply the usual test for analysing and proving aluminium, thus: a strongly brilliant mass is seen when hot, and if we moisten the powder with nitrate of cobalt and heat again, this time in the inner flame, the mass becomes blue. Other phenomena are seen during the influence of heat. Some stones, as stated, become pink on heating, but if the heating is continued too long, or too strongly, the stone is decoloured. Others, again, suffer no change, and this has led to a slight difference of opinion amongst chemists as to whether the colour is due to inorganic or organic matter. Heating also produces electricity, and the stone, and even splinters of it, will give out a curious phosphorescent light, which is sometimes yellow, sometimes blue, or green. Friction or pressure produces strong electrification; thus the stones may be electrified by shaking a few together in a bag, or by the tumbling of the powdered stone-grains over each other as they roll down a short inclined plane. The stones are usually found in the primitive rocks, varying somewhat in different localities in their colour; many of the Brazilian stones, when cut as diamonds, are not unlike them.

In testing, besides those qualities already enumerated, the crystalline structure is specially perfect and unmistakable. It is doubly refractive, whereas spinel and the diamond, which two it closely resembles, are singly refractive. Topaz is readily electrified, and, if perfect at terminals, becomes polarised; also the commercial solution of violets, of which a drop only need be taken for test, is turned green by adding to it a few grains of topaz dust, or of a little splinter crushed to fine powder.

The Beryl.

The beryl is a compound of silicates of beryllia and alumina, with the formula 3BeOSiO_{2} + Al_{2}O_{3},3SiO_{2}, or 3BeO,Al_{2}O_{3},6SiO_{2}. It differs very little indeed from the emerald, with the exception of its colour. In the ordinary varieties this is somewhat poor, being mostly blue, or a dirty or a greenish yellow; the better kinds, however, possess magnificent colour and variety, such as in the aquamarine, emerald, etc. The cleavage is parallel to the basal plane. Its lustre is sometimes resinous, sometimes vitreous, and it crystallises in the 2nd (hexagonal) system. It occurs in somewhat long, hexagonal prisms, with smooth, truncated planes, and is often found in granite and the silt brought down by rivers from granite, gneiss, and similar rocks. It is found in Great Britain and in many parts of Europe, Asia, and America, in crystals of all sizes, from small to the weight of several tons. The common kinds are too opaque and colourless to be used as gems and are somewhat difficult of fusion under the blowpipe, on the application of which heat some stones lose their colour altogether, others partly; others, which before heating were somewhat transparent, become clouded and opaque; others suffer no change in colour, whilst some are improved. In almost every case a slight fusion is seen on the sharp edges of fractures, which become smooth, lose their sharpness, and have the appearance of partly fused glass. The hardness varies from 7-1/4 to 8, the crystals being very brittle, breaking with a fracture of great unevenness. The better varieties are transparent, varying from that to translucent, and are called the "noble" beryls. Transparent beryl crystals are used by fortune-tellers as "gazing stones," in which they claim to see visions of future events.

The Emerald.

Considering the particular emerald which is a variety of beryl—although the name emerald in the trade is applied somewhat loosely to any stone which is of the same colour, or approaching the colour of the beryl variety—this emerald only differs chemically from the beryl, just described, in possessing an addition of oxide of chromium. In shape, crystallisation, fracture and hardness, it is the same, and often contains, in addition to the chromium, the further addition of traces of carbonate of lime, magnesia, and occasionally faint traces of hornblende and mica, which evidently result from its intimate association with the granite rock and gneiss, amongst which it is mostly found, the latter rocks being of a slaty nature, in layers or plates, and, like granite, containing mica, pyrites, felspar, quartz, etc.

Emeralds have been known from very early times, and are supposed to have been found first in the mines of ancient Egypt. They were considered amongst the rarest and the most costly of gems, and it was the custom, when conferring lavish honour, to engrave or model emeralds for presentation purposes. Thus we find Pliny describes Ptolemy giving Lucullus, on his landing at Alexandria, an emerald on which was engraved his portrait. Pliny also relates how the short-sighted Nero watched the fights of gladiators through an eye-glass made of an emerald, and in ancient times, in Rome, Greece, and Egypt, eye-glasses made of emeralds were much valued. Many of these, as well as engraved and carved emeralds, have been discovered in ruins and tombs of those periods.

The copper emerald is rare; it is a hydrous form of copper silicate, CuOSiO{2} + H{2}O, of a beautiful emerald green, varying from transparent to translucent. It exhibits double refraction, and is a crystallised mineral, brittle, and showing a green streak. This is less hard than the real emerald, is heavier, deeper in colour, and is usually found in crystals, in cavities of a particular kind of limestone which exists at Altyn-Tuebe, a hill in the Altai Mountains, in the Urals, and in North and Central America.

The Tourmaline.

The tourmaline is a most complex substance; almost every stone obtained has a different composition, some varying but slightly, with mere traces of certain constituents which other stones possess in a perceptible degree. Consequently, it is not possible to give the chemical formula, which might, and possibly would, be found but seldom, even in analyses of many specimens. It will therefore be sufficient to state the average composition, which is:—ferrous oxide, manganous oxide, potash, lime, boracic acid, magnesia, soda, lithia, and water. These form, roughly speaking, 25 per cent. of the bulk, the remainder being oxide of silicon and oxide of aluminium in about equal parts. It crystallises in the 2nd (hexagonal) system, with difficult cleavage and vitreous lustre.

It will naturally be expected that a substance of such complexity and variety of composition must necessarily have a corresponding variety of colour; thus we find in this, as in the corundum, a wonderful range of tints. The common is the black, which is not used as a gem. Next come the colourless specimens, which are not often cut and polished, whereas all the transparent and coloured varieties are in great demand. To describe adequately their characteristics with relation to light would alone require the space of a complete volume, and the reader is referred to the many excellent works on physics (optics) which are obtainable. This stone is doubly refracting, exhibiting extremely strong dichroism, especially in the blue and the green varieties. It polarises light, and when viewed with the dichroscope shows a remarkable variety of twin colours. It will be remembered that in Hogarth's "Rake's Progress," the youth is too engrossed in the changing wonders of a tourmaline to notice the entrance of the officers come to arrest him.



CHAPTER XV.

VARIOUS PRECIOUS STONES—continued.

Zircon.

Zircon appears to have been first discovered by Klaproth in 1789, in the form of an earth, and six years later he found that the stone hyacinth contained a similar substance, both having the formula, ZrSiO{4}, and both having as their colouring agent ferric oxide. There are several methods of obtaining the metallic element, zirconium; it is however with the silicate of zirconium that we have to deal at the moment. This is called zircon, ZrSiO{4}, or hyacinth when transparent or red, but when smoke-coloured, or colourless, it is the jargoon, or jarcon, and is found in silt and alluvial soils, limestone, gneiss, and various forms of schist, in India, Australia, the Urals, and certain parts of America. It is often combined with and found in juxtaposition to gold and certain varieties of precious stones. The lines of cleavage are parallel to the sides of the prism, and the crystals have an adamantine, or diamond lustre, varying from the completely opaque to the transparent. In some varieties the oxide of uranium is also present in traces. It crystallises in the 3rd (tetragonal) system, with indistinct cleavage. Its specific gravity varies from 4.70 to 4.88, according to the specimen and the locality.

This stone, like some of the others described, has a very wide range of colour, going through reds, browns, greens, yellows, oranges, whites, greys, blues from light to indigo, notwithstanding which it is somewhat difficult to imitate scientifically, though its composition of 33 per cent. of silica with 67 per cent. of zirconia (the oxide of zirconium), is practically all it contains, apart from the colouring matter, such as the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2, consequently it is untouched by a file, and so far, if one or perhaps two of the three qualities of colour, hardness, and specific gravity, are obtained in a chemically made zircon, the third is wanting. Under the blowpipe, zircons are infusible, but the coloured stones when heated strongly become heavier, and as they are contracting, their colour fades, sometimes entirely, which changes are permanent, so that as they possess the adamantine lustre, they are occasionally cut like a diamond, and used as such, though their deficiency in fire and hardness, and their high specific gravity, make them readily distinguishable from the diamond.

On exposure to light the coloured zircon becomes more or less decoloured; especially is this so in sunlight, for when the direct rays of the sun fall upon it, the colours fade, and for a moment or two occasional phosphorescence follows, as is the case when the stone is warmed or heated in a dark room. The stone appears to be very susceptible to brilliant light-rays, and in certain specimens which were split for testing, one half of each being kept excluded from light for purposes of comparison, it was found that sunshine affected them most; then brilliant acetylene gas, which was more effective still when tinted yellow by being passed through yellow glass. The electric arc was not so effective, but the electric light of the mercury-vapour lamp, though causing little change at the first, after a few hours' exposure rapidly bleached certain of the colours, whilst having no effect on others. Coal gas with incandescent fibre mantle was slightly effective, whilst the coal-gas, burned direct through an ordinary burner, affected very few of the colours, even after twenty-four hours' exposure at a distance of three feet. In all these cases, though the colours were slightly improved by the stones being kept for a time in the dark, they failed to recover their original strength, showing permanent loss of colour.

The Silicates.

The chief of these are the garnets, crystallising in the cubic system, and anhydrous. The garnet is usually in the form of a rhombic dodecahedron, or as a trisoctahedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They average from 40 to about 42 per cent. of silica, the other ingredients being in fairly constant and definite proportions. They are vitreous and resinous in their lustre and of great variety of colour, chiefly amongst reds, purples, violets, greens, yellows and blacks, according to the colouring matter present in their mass. There are many varieties which are named in accordance with one or more of their constituents, the best known being: (A) The iron-alumina garnet, having the formula 6FeO, 3SiO{2} + 2Al{2}O{3}, 3SiO{2}. This is the "precious" garnet, or almandine, sometimes called the "Oriental" garnet; these stones are found in Great Britain, India, and South America, and are deep red and transparent, of vitreous lustre. They get up well, but certain varieties are so subject to defects in their substance, brought about by pressure, volcanic action, and other causes, some of which are not yet known, that their quality often becomes much depreciated in consequence. This inferior variety of the iron-alumina garnet is called the "common" garnet, and has little lustre, being sometimes opaque. The perfect qualities, or almandine, as described above, are favourite stones with jewellers, who mount great quantities of them.

The second variety is the (B) lime-iron garnet, formula, 6CaO,3SiO{2} + 2Fe{2}O{3},3SiO{2}. The chief of this class is the melanite, sometimes dull, yet often vitreous; it is mostly found in volcanic rocks, such as tuff; this variety is very popular with jewellers for mourning ornaments, for as it is a beautiful velvet-black in colour and quite opaque, it is pre-eminent for this purpose, being considerably less brittle than jet, though heavier. Another variety is the "topazolite," both yellow and green. The "aplome" is greenish-yellow, yellowish-green, brown, and usually opaque. A further form of lime-iron garnet is the "pyreneite," first found in the Pyrenees Mountains, hence its name.

The (C) lime-chrome garnets—6CaO,3SiO{2} + 2Cr{2}O{3}, 3SiO{2}—the chief of which is "uwarowite." This is of a magnificent emerald green colour, translucent at edges and of a vitreous lustre. When heated on the borax bead it gives an equally beautiful green, which is, however, rather more inclined to chrome than emerald. This is an extremely rare stone in fine colour, though cloudy and imperfect specimens are often met with, but seldom are large stones found without flaws and of the pure colour, which rivals that of the emerald in beauty.

The fourth variety (D) is the lime-alumina garnet, its formula being—6CaO,3SiO{2} + 2Al{2}O{3},3SiO{2}. Like the others, it has a number of sub-varieties, the chief being the "cinnamon stone," which is one of great beauty and value when perfect. This stone is almost always transparent when pure, which property is usually taken as one of the tests of its value, for the slightest admixture or presence of other substances cloud it, probably to opacity, in accordance with the quantity of impurity existent. This variety is composed of the oxides of aluminium and silicon with lime. In colour it ranges from a beautiful yellowish-orange deepening towards the red to a pure and beautiful red.

"Romanzovite" is another beautiful variety, the colour of which ranges through browns to black. Another important variety is the "succinite," which gets up well and is a favourite with jewellers because of its beautiful, amber-like colour, without possessing any of the drawbacks of amber.

(E) The magnesia-alumina garnet—6MgO,3SiO{2} + 2Al{2}O{3},3SiO{2}—is somewhat rare, the most frequently found being of a strong crimson colour and transparent. This variety is called "pyrope," the deeper and richer tints being designated "carbuncle," from the Latin carbunculus, a little coal, because when this beautiful variety of the "noble" garnet is held up between the eyes and the sun, it is no longer a deep, blood-red, but has exactly the appearance of a small piece of live or glowing coal, the scarlet portion of its colour-mixture being particularly evident. The ancient Greeks called it anthrax, which name is sometimes used in medicine to-day with reference to the severe boil-like inflammation which, from its burning and redness, is called a carbuncle, though it is more usual to apply the word "anthrax" to the malignant cattle-disease which is occasionally passed on to man by means of wool, hair, blood-clots, etc., etc., and almost always ends fatally. A great deal of mystery and superstition has always existed in connexion with this stone—the invisibility of the bearer of the egg-carbuncle laid by a goldfinch, for instance.

(F) The manganese-alumina garnet—6MnO,3SiO_{2} + 2Al{2}O_{3},3SiO_{2}—is usually found in a crystalline or granular form, and mostly in granite and in the interstices of the plates, or laminae, of rocks called schist. One variety of this, which is a deep hyacinth in colour, though often of a brown-tinted red, is called "spessartine," or "spessartite," from the district in which it is chiefly found, though its distribution is a fairly wide one.

The Lapis-Lazuli.

The lapis-lazuli, sometimes called "azure stone," is almost always blue, though often containing streaks of white and gold colour, the latter of which are due to the presence of minute specks or veins of iron pyrites, the former and colourless streaks being due to free lime, calcite, and other substances which have become more or less blended with the blue colour of the stone. It has a vitreous lustre, crystallises in the 1st, or cubic system, and is a complex substance, varying considerably in its ingredients in accordance with the locality in which it is found, its matrix, and the general geological formation of the surrounding substances, which may, by the penetration of moisture, be brought to bear upon the stone, thus influencing to a great extent its chemical composition. So that we find the stone composed of about a quarter of its substance of alumina, or oxide of aluminium, silica to the extent of almost half, the remainder being lime, soda, sulphur, and occasionally traces of copper and iron. It is mostly found in granite and certain crystalline limestone rocks, in fairly large masses. It is of great antiquity, figuring extensively in ancient Egyptian history, both in its form as a stone and ground up into a pigment for the decoration of sacred and royal vessels and appointments. When so ground, it forms the stable and magnificent colour, genuine ultramarine, which is the finest and purest blue on the artist's palette, but owing to its extremely high price its use is not in very great demand, especially as many excellent chemical substitutes of equal permanence are obtainable at little cost.

The Turquoise.

The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour it is blue or greenish-blue, sometimes opaque, varying between that and feeble translucency, though it should be said that in all forms, even those considered opaque, a thin cutting of the stone appears almost transparent, so that the usual classing of it among the opaque stones must be done with this reservation. In composition it contains about 20 per cent. of water, about a third of its substance being phosphoric acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina, with small quantities of iron in the form of variously coloured oxides, with oxide of manganese. The great proportion of water, which it seems to take up during formation, is mostly obtained in the cavities of weathered and moisture-decomposing rocks. Its average formula may be said to be Al{2}O{3}P{2}O{5} + 5H{2}O, and sometimes Al{2}O{3} FeOP{2}O{5} + 5H{2}O. It must therefore follow that when the stone is heated, this water will separate and be given off in steam, which is found to be the case. The water comes off rapidly, the colour of the stone altering meanwhile from its blue or blue-green to brown. If the heat is continued sufficiently long, this brown will deepen to black, while the flame is turned green. This is one of the tests for turquoise, but as the stone is destroyed in the process, the experiment should be made on a splinter from it.

This stone is of very ancient origin, and many old turquoise deposits, now empty, have been discovered in various places. History records a magnificent turquoise being offered in Russia for about L800 a few centuries ago, which is a very high price for these comparatively common stones.

Owing to the presence of phosphorus in bones, it is not uncommon to find, in certain caves which have been the resort of wild animals, or into which animals have fallen, that bones in time become subjected to the oozing and moisture of their surroundings; alumina, as well as the oxides of copper, manganese and iron, are often washed across and over these bones lying on the cave floor, so that in time, this silt acts on the substance of the bones, forming a variety of turquoise of exactly the same composition as that just described, and of the same colour. So that around the bones there eventually appears a beautiful turquoise casing; the bone centre is also coloured like its casing, though not entirely losing its bony characteristics, so that it really forms a kind of ossified turquoise, surrounded by real turquoise, and this is called the "bone turquoise" or "odontolite."



INDEX

Adamantine lustre, 28 glimmering, 29 glinting, or glistening, 29 lustreless, 29 shining, 29 splendent, 29

Agate, 11

Almandine, 101

Amethyst, 11 oriental, 85 sapphire, 85

Amorphous stones and their characteristics, 23

Analysis, 5

Aplome, 101

Asters, or asteriated stones, 82, 87-91

Azure-stone, 103

Beryl, 10, 94 colours of, in dichroscope, 34

Beryllium, 10

Bezils, 66

Black stones, list of, 79

Blue sapphire, composition of the, 10 stones, list of, 77

Bone-turquoise, 106

Break, as opposed to cleavage, 19

Brilliant-cut stones, 66

Brown stones, list of, 76

Building up of crystals, 13

Burnt, or pinked topaz, 92

Cabochon-cut stones, 64 (the double), 65 (the hollow), 65

Carbonate series, 11

Carbuncle, 102, 103

Cat of Egypt, 89

Cat's eye stones, 82, 87-91 list of (see "Chatoyant Stones"), 78

Ceylonese cat's eye (see "Cat's eye")

Change of colour (not to be confused with "Play of colour" and "Opalescence," which see; see also "Fire"), 36

Characteristics of precious stones, 1, 3

Chatoyant stones, list of, 78

Chemical illustration of formation of precious stones, 8

Chloride of palladium in dichroscope, 34

Chrysoberyl, 88

Chrysolite, 11 ordinary, or "noble", 85 oriental, 85

Cinnamon stone, 102

Claims of precious stones, 4

Cleavage affecting tests, 43 and "cleavage" as opposed to "break", 19, 22

Colour, 26, 28, 30, 32

Colourless stones, list of, 75

Colours and characteristics of the various opals, 35, 36 of precious stones, list of, 75-79

Common garnet, 101 opal, 35

Composite crystals, 13

Composition of paste, or strass, for imitation stones, 71

Composition of precious stones, 7

Converted stones, 72

Corundum, 82

Crown portion of stones, 65, 66

Crystalline structure, physical properties, of 13

Crystallography, 14

Crystals, axes of symmetry, 15 groups of, 15, 16 planes of symmetry, 15 systems of, 16 (1) Cubic—isometric, monometric, regular, 16 (2) Hexagonal—rhombohedral, 16 (3) Tetragonal—quadratic, square prismatic, dimetric, pyramidal, 16 (4) Rhombic—orthorhombic, prismatic, trimetric, 16 (5) Monoclinic—clinorhombic, monosymmetric, oblique, 16, 17 (6) Triclinic—anorthic, asymmetric, 16, 17 treatment of, 14

Culasse portion of stones, 66

Cullinan diamond (see also "Stars of Africa"), 22, 64, 68, 80

Cutting of precious stones, 3, 4, 62

Cymophane, 90

Definition of a precious stone, 1

Diamond, characteristics of the, 80 composition of the, 10 (sapphire), 86 unique, 10 (zircon), 99

Diaphaneity, 26, 28

Diaphanous stones, 28

Dichroscope, 33 how to make a, 33 how to use a, 34

Dimorphism in precious stones, 25

Double cabochon-cut stones, 65 refraction (see "Refraction")

Doublets, 72

Electric and magnetic influences, 57 experiments with precious stones and pithball and electroscope, 57 experiments with tourmaline, 58, 59

Emerald, 10, 11, 95, 96 oriental, 85

En cabochon-cut stones, 64

Experiments to show electric polarity, 58, 59

Facets in stones, description of the, 67, 68

Feminine stones, 85

Fire in stones (see also "Change of Colour," "Opalescence," and "Play of Colour"), 36, 37

Fire opal, 35

Flame-coloured stones, list of, 76

Flaws, 63

Formation of precious stones, 5, 8 chemical illustration of, 8, 9

Garnet, 11, 100

Garnets (A) iron-alumina (called also almandine and precious or oriental garnet), 101 sub-variety, common garnet, 101 (B) lime-iron, 101 sub-variety aplome, 101 melanite, 101 pyreneite, 101 topazolite, 101 (C) lime-chrome, 101, 102 sub-variety uwarowite, 101, 102 (D) lime-alumina, 102 sub-variety cinnamon stone, 102 romanzovite, 102 succinite, 102 (E) magnesia-alumina, 102, 103 sub-variety carbuncle, or anthrax, 102, 103 noble, 103 pyrope, 102 (F) manganese-alumina, 103 sub-variety spessartine, or spessartite, 103

Girdle portion of a stone, 66

Glimmering, in lustre, definition of, 29

Glinting, or glistening in lustre, definition of, 29

Goutte de suif-cut stones, 65

Great Mogul diamond, 64

Green stones, list of, 78

Groups of crystals (see "Crystals")

Hardness, physical properties of, 39 table of, 39, 40, 41

Heat indexes, 54 physical properties of, 52

Hollow-cabochon, 65

Hyacinth, ordinary (a form of zircon), 85, 98 oriental, 85

Hyalite (opal), 35

Hydrophane (opal), 35

Imitations and tests of precious stones, 70

Indigo sapphires, 86

Ink sapphires, 85

Iridescence, and cause of, 37, 38

Iron-alumina garnets, 101

Jacinth, oriental, 85

Jarcon, or jargoon, 98

Koh-i-nur, 64

Lapis-lazuli, 103

Light, physical properties of, 26

Lime-alumina garnets, 102 cinnamon stone, 102 romanzovite, 102 succinite, 102

Lime-chrome garnets, 101, 102 uwarowite, 101, 102

Lime-iron garnets, 101 aplome, 101 pyreneite, 101 topazolite, 101

List of stones according to colour, 75-79 hardness, 39-41 specific gravity, 48-50

Lustre, 26, 28

Lustreless, definition of, 29

Lynx-eye stones, 87

Magnesia-alumina garnets, 102, 103 carbuncle, or anthrax, 102 noble, 103 pyrope, 102

Magnetic and electric influences, 57-61

Malachite, 11

Manganese-alumina garnets, 103 spessartine, or spessartite, 103

Masculine stones, 85

Melanite, 101

Menilite (opal), 36

Metallic-lustre stones, 28, 29

Mohs's table of hardness, 39-41

Noble garnet, 103 or precious opal, 35

Non-diaphanous stones, 28

Odontolite, 106

Olivine corundum (see "Chrysolite"), 85

Opal, 11 varieties of, 35, 36

Opalescence (not to be confused with "Change of Colour" and "Play of Colour," which see; see also "Fire"), 36, 37

Oriental amethyst, 85 cat's eye (see "Cat's eye") emerald, 85 garnet, 101 topaz, 85

Origin of precious stones, 7

Paste, or strass, for imitation stones, composition of, 71

Pavilion portion of cut stones, 66

Pearly-lustre stones, 28, 29

Peridot (see "Noble Chrysolite"), 85

Pink-coloured stones, list of (see also Red), 77

Pinked topaz, 92

Phosphorescence, 26, 30

Physical properties:— A.—Crystalline structure, 13 B.—Cleavage, 19 C.—Light, 26 D.—Colour, 32 E.—Hardness, 39 F.—Specific gravity, 45 G.—Heat, 52 H.—Magnetic and electric influences, 57

Play of colour (not to be confused with "Change of Colour" and "Opalescence," which see; see also "Fire"), 36, 37

Pleochroism, 33

Polarisation, electric, 58, 59 of light, 26, 27

Polariscope, 27, 28

Polishing precious stones, 3, 4

Polymorphism in precious stones, 25

Precious, or noble opal, 35

Pseudomorphism in precious stones, 23, 24

Pyreneite, 101

Pyro-electricity, development and behaviour of, 58-60

Pyrope, 102

Qualities of precious stones, 1, 3

Red and rose-coloured stones, list of (see also Pink), 76, 77

Reflection of light, 26, 28

Refraction of heat, 52-55 light, 26, 27

Reproduction of crystalline form, 20, 21

Resinous lustre stones, 28, 29

Rock-crystal, 11

Romanzovite, 102

Rose-coloured stones (see Red, above), 76, 77

Rose, or rosette-cut stones, 65

Rothschild's testing solution, 73

Ruby, characteristics of, 83 composition of, 10

Sapphire, amethyst, 85 and its varieties, 84, 85 cleared, 86 diamonds, 87 indigo, 86 ink, 85 the blue, composition of, 10, 85 water, 86

Semi-diaphanous stones, 28

Shining, in lustre, definition of, 29

Silica group, composition of the, 11

Silicates, 100

Silky-lustre stones, 28, 29

Single-refraction (see "Refraction")

South African diamond (see "Cullinan Diamond")

Specific gravity, 45

Splendent, in lustre, definition of, 29

Splitting of the Cullinan diamond, 22

Star-portion of stones, 65

Stars of Africa (see also "Cullinan Diamond"), 22, 64, 68

Starting or splitting of stones on cleavage planes, 23

Step-cut stones, 66

Stones arranged according to colour, 75-79 hardness, 39-41 specific gravity, 48-50

Strass for imitation stones, composition of, 71

Sub-metallic in lustre, definition of, 29

Sub-translucent stones, 28

Sub-transparent stones, 28

Succinite, 102

Synthesis, 5

Systems of crystals (see "Crystals")

Table-cut stones, 65

Tallow drops, 65

Teeth of stone, 65

Testing by crystalline structure, 17 hardness, 40, 43 with needles, 41 gems by dichroscope, 33, 34 solution (Rothschild's), 73

Tests of precious stones (general), 70

Topaz, 11, 91 colours of, in dichroscope, 34 oriental, 85

Topazolite, 101

Tourmaline, 96, 97 electric experiments with, 58, 59

Translucent stones, 28

Transmission of heat, 52-56 light, 26

Transparent stones, 28

Trap-cut stones, 66

Tri-morphism in precious stones, 25

Triplets, 72

Turquoise, 104 (bone), 106 composition of the, 11 odontolite, 106

Uwarowite, 101, 102

Violet stones, list of, 78

Vitreous-lustre stones, 28, 29

Water-sapphires, 86

White (paste) stones, 71 stones, list of, 75

Yellow stones, list of, 76 topaz, 92

Zircon, 10, 98 diamonds, 99

Zirconium, 10

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S. E.

THE END