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CHAPTER V.

ARTIFICIAL FORMATION OF PRECIOUS STONES.

A clear distinction must be made between the imitation of a precious stone and its actual reproduction or formation by artificial methods. In the former case we simulate the appearance of the natural substance by means of some product or preparation, which may be (and generally is) widely different in chemical composition and even in many physical properties. In the latter case we form the very mineral which Nature has formed, endowed with all its chemical and physical characters, but not necessarily produced by processes identical with those of Nature. A few examples of the true reproduction of precious stones will serve to explain the distinction pointed out with sufficient exactitude.

Take the case of the ruby and sapphire, varieties of crystallized alumina or corundum. If, by the aid of the intense heat of the oxyhydrogen blowpipe pure alumina, with traces of chromium oxide or other colouring oxide, be fused, we get a sapphire or ruby glass, having a hardness and density much less than those belonging to crystals of alumina. But by prolonging the time of cooling or by producing the alumina from some of its compounds during the heating, a portion of the product will crystallise in forms identical with those of the natural stone, and having the density of 4 and the hardness 9. For some time the specimens made were small in size and poor in colour and brilliancy, but the product was identical with native corundum. Now there have been, among the large numbers of artificially-prepared rubies, some of several carats in