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Sorry, this answer is incorrect. Corundum is actually the second hardest mineral known. Due to corundum's hardness, it is commonly used as an abrasive in machining and in sandpaper. Chemically, corundum is aluminum oxide. The oxygen atoms in corundum are arranged in a hexagonal close-packing, with the smaller aluminum atoms occupying 2/3 of the octahedral gaps. Corundum comes in different colors and shades depending on impurities present; as gemstone it is also known as red ruby or blue sapphire.
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