Corundum

Corundum, mineral consisting of aluminum oxide, Al₂O₃. Next to the diamond, corundum is the hardest natural substance, with a relative hardness of 9 and a specific gravity of about 4. Corundum crystallizes in the hexagonal system (see Crystal). Gem-quality transparent crystals of the mineral form sapphires and rubies. Common corundum, which occurs either in a crystallized or granular form, is usually gray, grayish-blue, or brown; the black granular variety, which contains varying amounts of hematite and magnetite, is called emery. Common corundum and emery are both used as abrasives. Synthetic corundum, sold under various trade names, is also used as an abrasive. Synthetic rubies and synthetic sapphires have also been made. See Abrasive; Alumina; Ruby; Sapphire.