These stones theoretically are composed of pure alumina, or aluminum, 53.2, and oxygen, 46.8. This oxide of aluminum, or alumina, is known in its natural state as corundum, a name derived from the Hindoo word " kurand," and the two stones ruby and sapphire are frequently spoken of as the " corundums." It is called ruby when it is red, and sapphire in any other color.
Comparatively little is found in the perfectly crystallized transparent condition necessary for gem purposes. Quantities of this mineral are unfit for jewels. This common corundum lacks transparency and color. The colorless transparent, called white sapphire, is the purest. The colored transparent varieties contain between one and two per cent, each of silica and oxide of iron, and it is supposed that the fine color of the Burmah rubies is due to the presence of a very small quantity of oxide of chromium. The common corundum contains a much greater proportion of impurities with the alumina. On account of its hardness it is useful for mechanical purposes. The best of it is used for bearings in fine machinery. Jewels for watches have long been made from it, and it is now used also in the manufacture of electrical supplies.
Various names are given to the mineral according to the color. The blood-red is invariably called ruby; blue to colorless is always known as sapphire; and all the other colors are usually termed fancy sapphires, but separate names are often attached to them, as follows : Violet, Oriental amethyst; green, Oriental emerald; yellow, Oriental topaz; yellowish red, Oriental hyacinth; light bluish green, Oriental aquamarine.
Although the crystallization of the different corundums is