Sapphire

Sapphire, the crystalline mineral form of the sesquioxide of aluminium (A1₂O₃). It crystallises in the Hexagonal system, but its crystals, as usually found in alluvial deposits, are water-worn. It varies in colour, being black and opaque in the Impure ferruginous variety known as emery, reddish-brown or white and opaque in corundum, red and transparent in the 0riental rnby (q.v.), violet in Oriental amethyst, colourless in lux sapphire, and blue in the variety to which the name sapphire is popularly restricted. This blue variety is dichroic, the velvety cornflower blue of a fine stone being resolved by the dichroiscope into ultramarine-blue and yellowish-green. Its colour may be due to cobalt-oxide, which is always used in imitating it. Its specific gravity is 3.9 to 4.1, and its hardness is 9 in Von Mohs' scale, but blue sapphire is harder than emery, corundum, or ruby. It is unaffected by acids, but is fusible with difficulty in borax or microcosmic salt, forming a clear bead. Sapphires occur in the basalts of the Rhine Valley; rarely in North Carolina, where coarse corundum is abundant; in Victoria and New South Wales, associated with gold; in Siam; with the ruby in Burma; but in the finest quality in Ceylon, in river sand. Some of these Cingalese stones are cloudy and when cut en cabochon, i.e. hemispherically, exhibit a six-rayed star or asterias. These are termed star-sapphires. The sapphire has been formed artificially, the most successful process being that of MM. Fremy and Fell in 1878.