A trigonal mineral, Al₂O₃; hematite group; forms hexagonal prisms with basal and rhombohedral parting; red (ruby), blue (sapphire), green (oriental emerald), reddishbrown, white, or gray; defines 9 on the Mohs hardness scale; in nepheline syenite pegmatites and placer deposits. Emery is granular corundum mixed with magnetite and spinel. Synthetic corundum made from bauxite together with other manufactured abrasives have largely replaced natural materials.