Any of several common, widespread aluminum or calcium silicate minerals occurring in two internally isomorphic series, (Mg, Mn, Fe)3Al2Si3O12 and Ca3(Cr, Al, Fe)2Si3O12, generally crystallized, often embedded in igneous and metamorphic rocks, and colored red, brown, black, green, yellow, or white and used both as gemstones and as abrasives.
A brittle and more or less transparent usually red silicate mineral that has a vitreous luster, occurs mainly in crystals but also in massive form and in grains, is found commonly in gneiss and mica schist, and is used as a semiprecious stone and as an abrasive.