corundum

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corundum Translucent to transparent mineral, aluminium oxide (Al2O3). It is found in igneous, pegmatitic and metamorphic rocks, occurring as pyramidal or prismatic crystals in the rhombohedral class and as granular masses. It is the hardest natural substance after diamond. Gemstone varieties are sapphire and ruby. It is an industrial abrasive. Hardness 9; r.d. 4.

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