GEMS (FR., Gemmes, Pierres fines ; Gut., Gemmen, Edelsteine).
The term " gems" or " precious stones," is applied to a very limited number of minerals con spicuous for their brilliancy and beauty of colour, and for their rarity and durability. They occur as druses in veins and fissures in the earth's crust, as geodes in the igneous rocks, and as accidental or accessory crystals iu the older metamorphic formations. They are never met with in great profusion, and only a small proportion of those found possess purity and brilliancy in the highest degree. Consequently, their value is subject to but slight changes, and a remunerative market is always ready for them.
In this article, will be described only the most esteemed gems, numbering less than a dozen ; the description will include their chemical and physical characteristics, their geographical and geological distribution, the modes of obtaining them, and such statistics of their production as may be available.
Artificial Gems.—The fabrication of compounds in imitation of gems has assumed the importance of an industrial art, and is prosecuted with skill and capital. It is carried on principally in France, in the Jura, Franche-Comte, the communes of St. Claude, Septmoncel la Meure, les Molunes, and surrounding districts. The basis of these imitation products is a pure, fusible, brilliant, transparent and dense variety of glass, termed "paste," or strass, appropriately tinted by the addition of metallic oxides. The proportions (by weight) of the ingredients of this paste are :— Pure pulverized silica, 45.7 parts ; pure dry carbonate of soda, 22 8 ; calcined borax, 7.6 ; saltpetre, pure red-lead (minium), 11.8. The source of the silica is rock-crystal ; flint contains impurities. Each ingredient is separately reduced to an impalpable powder ; they are then intimately mixed in due proportion, and placed in a Hessian crucible heated by charcoal ; the temperature is raised gradually till fusion has commenced, and is then maintained with the utmost uniformity for 20-30 hours ; they are finally cooled very slowly. The density and beauty of the product depend upon the regularity of the fusion, the intimacy of the combination, and the slowness of the cooling. The cooled compound, without any further treatment, is cut up, polished, set and foiled, in imitation of the diamond. Counterfeits of the other gems require the addition of pigments, in which some variety prevails. Thus imitation topaz is made by fusing 40 parts of glass of antimony, and 1 of purple of Cassius, with 1000 of the paste. From this compound at its opaque stage, ruby may be simulated, by melting 1 part with 8 parts of paste, in a Hessian crucible, for 30 hours, iu a glass-furnace ; the product is a yellowish crystal, which, remelted by the blowpipe, yields a mass resembling the finest ruby. Another colouring agent for topaz is 1.59 per cent. of oxide of
uranium ; and another for ruby is 5 parts of peroxide of manganese, and a trace of purple of Cassius, to 1000 of paste. Emerald is imitated by 1000 of paste, 8 of oxide of copper, and of oxide of chromium ; or, 0•53 per cent. of oxide of iron. For sapphire, mix 1000 of paste and 15 of oxide of cobalt ; or, 0.106 per cent. of carbonate of cobalt. Spurious emeralds, totally different from the above, haie recently appeared in the market. Bryce Wright states their composition to be :—Silica 35-70; lime, 41.66; soda and potash, ; " beryelia" (? beryllium, or berzelin), ; with traces of iron, chromium and lead ; alumina is not present. They are readily known by their high sp. gr.-3•402—and by their optical properties ; but their colour exceedingly well imitates the true gem, and they are flawed with remarkable faithfulness. None of the above compounds display the optical properties, the hardness, the sp. gr., nor the chemical constituents of the true gems. Another system of fraud, more difficult to discover, is the facing of pieces of worthless transparent material with thin slices of the real gem, affixed by means of an invisible cement ; but this is not commonly practised. (See Glass.) In quite another category from the above-described tricks, intended only to deceive the eye, and thus to palm off rubbish at a very fictitious value, must be placed recent discoveries permitting the synthetic production of several gems, differing in no essential respect from their natural prototypes. The first success of the kind was achieved by Frdmy and Foil, in the artificial formation of ruby and sapphire, differently coloured and crystallized forms of corundum. The process is as follows : A fusible aluminato is formed by calcining equal weights of alumina and red-lead in a double fire clay crucible at a bright-red heat for 20 days. During the operation, the lead salts attack the silica of the crucible. On cooling, two layers of matter are found : one is vitreous, and composed chiefly of silicate of lead ; the other is crystalline, and consists of alumina or corundum. To produce ruby-coloured corundum, 2-3 per cent. of bichromate of potash is added to the ingredients in the crucible ; the blue of sapphires is obtained by using small quantities of oxide of cobalt and biohromate of potash. Another method of producing rubies is by calcining equal weights of alumina and fluoride of barium, with 2-3 per cent. of bichromate of potash, in a covered crucible. In these manners, are produced large masses of a substance having all the hardness of natnral ruby, and probably capable of wide industrial application, even though it should not possess the brilliancy of true gems.