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 SiO4, or (AlF)2SiO4. 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 3BeOSiO2 + Al2O3,3SiO2, or 3BeO,Al2O3,6SiO2. 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, CuOSiO2 + H2O, 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-Tübe, a hill in the Altai Mountains, in the Urals, and in North and Central America.