Corundum

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Corundum
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
(+.009,-.005)
Birefringence 0.008-0.010
Dispersion Low, 0.018
Specific gravity 4
Lustre Vitreous to Subadamantine
Pleochroism Weak to moderate

Corundum.jpg
Corundum is an aluminum oxide that occurs in every color of the rainbow. It is allochromatic. When it is red it is termed a ruby. When it occurs in any other color it is termed a 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 today for rubies are Burma and Madagascar. The major source for sapphire is Madagascar. Other locals include Pakistan, Afghanistan, India, East Africa and Yogo Gulch in Montana.

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