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PRECIOUS STONES.

crowd of stones, practically identical in composition, but presenting great diversity in colour and optical properties. All these varieties belong however to the mineral species corundum, the French corindon, and consist of crystallised alumina, the oxide of the metal aluminium. From the mineralogical, or rather from the physical point of view, the colour of these stones is of no account, while chemistry has not as yet succeeded in discovering much concerning the causes of the variations of colour which determine the very different values set upon different specimens of corundum. That there are small quantities of magnesia, oxide of iron and silica in rubies and sapphires of all hues, has been ascertained, but this fact does not furnish the clue to the cause of the blue of the sapphire or of the red of the ruby. That certain chromium compounds impart a red hue to certain artificial preparations, both crystallised and vitreous, of alumina, will not count for much in the absence of proof that all rubies contain chromium. That iron is the cause of the dark colour of emery and other impure corundum is, however, certain: indeed some specimens of emery contain half their weight of iron oxide.

Coloured corundums, when strongly heated, generally change their hue, pale blue and pale yellow stones becoming colourless, and violet stones retaining only the red constituent of their original colour. In Ceylon the native dealers frequently offer for sale specimens exhibiting a beautiful pink or rose colour which is not natural, but has been produced by "firing" inferior corundums of the purple variety or oriental amethyst.

Corundum always occurs in crystals or is at least crystalline; the forms are six-sided prisms or pyramids belonging to the hexagonal (rhombohedral) system. The lustre is vitreous except on the basal planes which are often pearly. The six-rayed star seen in many cloudy sapphires and rubies, especially when cut en cabochon with the summit of the curved surface lying in the direction of the principal axis of the prism, is due to the peculiar intimate structure