Corundum, aluminum oxide, is a common and widespread mineral that is used as an abrasive because of its great hardness. Colored and transparent corundum is used as a gemstone. The red variety is called ruby; all other colors are termed sapphire. Ruby occurs in fine crystals in many localities. At Longido in Tanzania the ruby crystals are embedded in a dense, bright green chrome-rich variety of the mineral zoisite. This combination is unique to this locality.