Exceptionally hard mineral, crystallized alumina, used, when transparent, as gem.
Native aluminum oxide, Al2O3, the hardest naturally occurring mineral known apart from diamond (corundum rates 9 on the Mohs’ scale of hardness); lack of cleavage also increases its durability. Its crystals are barrel-shaped prisms of the trigonal system. Varieties of gem-quality corundum are ruby (red) and sapphire (any color other than red, usually blue). Poorer-quality and synthetic corundum is used in industry, for example as an abrasive.
Corundum forms in silica-poor igneous and metamorphic rocks. It is a constituent of emery, which is metamorphosed bauxite.