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Chemical composition Al2O3
Crystal system Trigonal
Habit Prismatic, tabular
Cleavage None; twinned stones may show parting
Fracture Conchoidal
Hardness 9
Optic nature Uniaxial -
Refractive index 1.762-1.770
Birefringence 0.008-0.010
Dispersion Low, 0.018
Specific gravity 4
Lustre Vitreous to Subadamantine
Pleochroism Weak to moderate

Corundum is an aluminum oxide that occurs in every color of the rainbow. It is allochromatic. When it is red, it is termed ruby. When it occurs in any other color, it is termed sapphire. There are two primary ways that corundum is formed. One is the metamorphosis of limestone and the other is an igneous occurrence in rocks lacking in silica. Since corundum is so hard, it is very resistant to weathering. Therefore, it accumulates in placer gravels. Placer deposits in Sri Lanka at Ratnapura have been mined since before the time of Buddha. The major sources for rubies today are Burma and Madagascar. The major source for sapphire is Madagascar. Other locations include Pakistan, Afghanistan, India, East Africa and Yogo Gulch in Montana.