alumina
Oxide of aluminium, widely distributed in clays, slates, and shales. It is formed by the decomposition of the feldspars in granite and used as an abrasive. Typically it is a white powder, soluble in most strong acids or caustic alkalis but not in water. Impure alumina is called ‘emery’. Rubies, sapphires, and topaz are corundum gemstones. It is the chief component of bauxite.
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