Aluminium oxide. The name is probably derived from an old Indian word, corund, which referred to an unknown mineral or gem.
Crystal system Trigonal.
Appearance It occurs in semi opaque masses similar to whitish or grayish vein quartz, but also in distinct, prismatic or tapered crystals, with close transverse striations, some of which resemble elongated bi pyramids. Often opaque or translucent, corundum can be partially or perfectly transparent. All the colours of the spectrum are possible, from red to yellow, green, blue, and violet; in addition, the stones may be pink, gray, black, or colourless and all the shades between. Brightly colored, transparent, translucent, or semi opaque varieties make highly aesthetic and valuable gems. Because of its hardness and resistance to chemical attack, corundum is often found in- alluvial deposits in the form of pebbles that retain clear indications of their original crystal shapes.
Physical properties Corundum has a hardness of 9, the highest in the mineral world after diamond. The density is approximately 4.0 g/cm3_ The refractive indices are about ne 1.760, nw 1.769. Parting parallel to the basal plane is sometimes visible, with an appearance of cleavage.
Genesis It is formed by contact metamorphism between alumina-rich magmas (and related pegmatite's) and lime stone, or by regional metamorphism of alumina-rich, silica poor rocks.
Occurrence The least attractive variety of corundum, known as emery (usually a corundum-magnetite mixture) and used as an abrasive, is mainly found in Greece, the United States, and Australia. The gem varieties come chiefly from Sri Lanka, Thailand, Cambodia, Burma, and Australia, with smaller deposits in India, Tanzania, and the United States.