corundum
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Corundum. Credit: Mineral Information Institute
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A crystalline (rhombohedral) form of aluminum oxide (alpha-Al2O3). The corundum structure has close-packed oxygen with aluminum in octahedral holes. Mineral varieties of corundum include ruby, sapphire, and emery. Artificial corundum, or beta-alumina, is made by calcining bauxite. Corundum is the hardest natural substance known after diamond (Mohs hardness 9) and is also chemically inert and resistant to corrosion. It is used extensively as an abrasive and in bearings.
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