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, mineral, aluminum oxide, Al2O3. The clear varieties are used as gems and the opaque as abrasive materials. Corundum occurs in crystals of the hexagonal system and in masses. It is transparent to opaque and has a vitreous to adamantine luster.
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fine-grained rock in which the corundum is closely interlocked with such minerals as magnetite, sulfides, and brittle mica. Containing up to 60 percent corundum, emery is one of the natural abrasive materials. Emery has a hardness of 7–8 on the mineralogical scale. It is found chiefly in marble in the form of lenses, stocks, nests, and veins. Less frequently emery occurs in the form of xenolith in gabbro, norite, and granite or at the contact between these rocks and enclosing rock.