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Stones (PRECIOUS lapis lazuli)

PRECIOUS lapis lazuli, crocidolite, labradorite, moonstone, avanturine, and malachite. To the list of precious stones there should be added two substances of animal originpearls and red coral-and perhaps also amber, a comparatively rare and valuable fossil resin. When luster and sparkle are the principal qualities to be revealed, as in the case of the diamond, the surface is most favorably cut into numerous plane facets as either brilliant or rose cut stones.

When color is the more important quality of the stone it may, if plane surfaces are wanted, be step or table cut. Such stones also, and translucent and opaque stones, may be cut en cabochon-i. e., with curved or rounded surfaces.

One of the most important qualities of a precious stone is its hardness, as on that property depends its power of resisting wear and of keeping the brilliance of its polished surface. It is a property of great constancy, moreover, and in many cases affords a ready means of determining the nature of a stone under examination. Of all known substances diamond is the hardest, and representing it, according to Mohs's scale, by 10, the following is the relative hardness of several of the more important of the precious stones: diamond, 10.0; sapphire, 9.0; ruby, 8.8; chrysoberyl, 8.5; spinel, 8.0; topaz, 8.0; aquamarine, 8.0; emerald, 7.8; zircon, 7.8; tourmaline, 7.5; amethyst, 7.0; moonstone, 6.3; turquoise, 6.0; opal, 6.0. The value of the precious stones produced in the United States in 1919 was $123,046. The largest in value was corundum, $40,304, followed by turquoise, $30,537; quartz, tz, $19,078, 19,078, and tourmaline, $18,642. See DIAMOND; GEM.

Artificial Precious Stones. -Numerous attempts have been made by eminent investigators to produce artificial precious stones by means of intense heat and pressure and by electrical action; but hitherto these efforts have failed of practical success. In an important memoir published by Sainte Claire Deville and Caron in 1858, they describe various processes by which they obtained small crystals of corundum, ruby, sapphire, etc.

By the action of the vapors of fluoride of aluminum and boracic acid on one another, they obtained crystals which, in hardness and in optical properties, resembled natural corundum. When a little fluoride of chromium was added a similar process yielded violet-red rubies; with rather more fluoride of chromium blue sapphires were yielded; and with still more green corundum was obtained.

A mixture of equal equivalents of the fluorides of aluminum and glucinum, when similarly acted on by boracic acid, STONINGTON yielded minute crystals of chrysoberyl.

The action of fluoride of silicon on zirconia yields small crystals of zircon, and by the action of silica on a mixture of the fluorides of aluminum and glucinum hexagonal plates of extreme hardness were obtained, which in some respects resembled emerald.

In subsequent researches Becquerel, by the use of electric currents of high tension succeeded in obtaining opals, etc., from solutions of silicates. Among the most successful of experimenters in this direction was Ch. Feil, of Paris, who successfully crystallized alumina, and by the introduction of coloring matter produced sapphires and rubies identical in hardness and composition, but not in brilliance, with the natural stones. M.

Feil also succeeded in preparing true crystals of spinel, and a blue lime spinel of great hardness, hardness, but which were glassy rather than crystalline in structure.

Imitations of precious stones consist of a soft, heavy flint-glass called strass, or paste, appropriately colored, and they may readily be distinguished, among other peculiarities, by their great softness. Fraudulent combinations are made by cementing thin plates of precious materials over, and sometimes under, a body of valueless glass.