A compound of two parts aluminum and three parts oxygen which occurs naturally as corundum. Alumina is the base of aluminous salts, a constituent of feldspars, micas, etc., and the characterizing ingredient of common clay, in which it exists as an impure silicate with water, resulting from the erosion of other aluminous minerals. In a hydrated form it is bauxite. Alumina is used in aluminum production and in abrasives, refractories, ceramics, and electrical insulation.