ETYM Latin alumen, aluminis. Related to Alum.
Oxide of aluminum.
Any of various forms of aluminum oxide occurring naturally as corundum; SYN. aluminum oxide, aluminium oxide.
Or corundum; Al2O3 oxide of aluminum, 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 and sapphires are corundum gemstones.