Corundum
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Corundum is the crystalline form of aluminium oxide (Al2O3) and one of the rock-forming minerals. Corundum is naturally called sapphire. The word corundum comes from the Hindu kurand.
Corundum has a hardness of 9 on the point Mohs' scale, a specific gravity of 4.00 and a hexagonal crystal structure. The oxygen atoms are arranged in a hexagonal close packing, with the smaller aluminum atoms occupying 2/3 of the octahedral gaps.
Due to corundum's hardness, it is commonly used in as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 7.5. Diamond is harder, but much more expensive.
See also: List of minerals