CORUNDUM Al2O3
Aluminium oxide. Then name is probably derived from and old Indian word, corund, which referred to and unknow mineral or gem.
Crystal system Trigonal.
Appearance It occurs in semiopque 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 bipyramids. Often opaque of translucent, corundum can be partially or perfecity transparent. All the colors of the spectrum are possible, from red to yellow, green, blue, and violet; in addition, translucent, or semiopaque varieties make highly aesthetic and valuable gems. Because of its hardness and resistance to chemical attack, corundum is often found in alluvial deposits in then form of pebbles that retain clear indications of their original crystal shapes.
Physical properties Corundum has a hardness of 9. then highest in the mineral world after diamond. The density is approximately 4.0 g/cm3 . The refractive indices are about 1.76,1.769 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 pegmatites) and limestone, or by regional metamorphism of alumina-rich, silica poor rocks.
Occurrence The least attractive variety of corundum, know 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, Tranzania, and then United States.