Corundum

(ko-rün'dum.) [Hind.] (Min.) A mineral which, though composed of nearly pure alumina, is of great sp. gr.- abt. 4 times that of water-and of re mark able hardne , being inferior in this respect only to the diamond. Mineralogists regard the sapphire as a variety of C., and along with it the gems popularly known as Oriental Ruby, Oriental Topaz, Oriental Emerald, and Oriental Amethyst; but the name C. is more usually limited to the coarser varieties, to which it is applied by the natives of India. These, instead of exhibiting the brilliancy of gems, are in general of a dull and muddy appearance, and the crystals-which are usually six-sided prisms, and six-sided pyramids-are externally dull and rough. The color is various, often green, blue, or red, inclining to gray. [s. 232]