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