Corundum

Corundum, a mineral consisting essentially of mere alumina, yet of great specific gravity—about four times that of water—and of remarkable hardness, being inferior in this respect only to the diamond. Mineralogists regard the Sapphire (q.v.) as a variety of corundum, and along with it the gems popularly known as Oriental Ruby, Oriental Topaz, Oriental Emerald, and Oriental Amethyst; but the name corundum is more usually limited to the coarser varieties. 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 colour is various, often green, blue, or red, inclining to gray. Corundum is found in many parts of the world, and has long been used in India for polishing all gems except the diamond, which is too hard for it. Emery (q.v.), so well known as a polishing substance, is a variety of corundum.