The Chemistry, Properties and Tests of Precious Stones Part 6

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 pa.s.s 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 ma.s.ses 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 superst.i.tions 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 G.o.ddess Isis. This, therefore, may be considered one of their sacred stones, whilst there is some a.n.a.logy 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 superst.i.tions 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 l.u.s.tre 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 gla.s.s, 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 gla.s.s 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 l.u.s.tre 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 gla.s.s 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 a.n.a.lysing and proving aluminium, thus: a strongly brilliant ma.s.s is seen when hot, and if we moisten the powder with nitrate of cobalt and heat again, this time in the inner flame, the ma.s.s 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 phosph.o.r.escent 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 l.u.s.tre 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 gla.s.s.

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 "n.o.ble" 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 a.s.sociation 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-gla.s.s made of an emerald, and in ancient times, in Rome, Greece, and Egypt, eye-gla.s.ses 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-Tube, 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 const.i.tuents 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 a.n.a.lyses 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 l.u.s.tre.

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 s.p.a.ce 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 l.u.s.tre, 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 l.u.s.tre, 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 phosph.o.r.escence 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 pa.s.sed through yellow gla.s.s. 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 l.u.s.tre and of great variety of colour, chiefly amongst reds, purples, violets, greens, yellows and blacks, according to the colouring matter present in their ma.s.s. There are many varieties which are named in accordance with one or more of their const.i.tuents, 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 l.u.s.tre. 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 l.u.s.tre, being sometimes opaque. The perfect qualities, or almandine, as described above, are favourite stones with jewellers, who mount great quant.i.ties 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 cla.s.s 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 l.u.s.tre. 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 quant.i.ty 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 "n.o.ble" 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 pa.s.sed on to man by means of wool, hair, blood-clots, etc., etc., and almost always ends fatally. A great deal of mystery and superst.i.tion 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 "spessart.i.te," 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 l.u.s.tre, 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 ma.s.ses. 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 subst.i.tutes 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 cla.s.sing 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 quant.i.ties 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 800 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."