Alumina
. Alumina or Aluminum Oxide, an oxide found in nature as the minerals corundum, diaspore, gibbsite, and most commonly, bauxite, an impure form of gibbsite. It is the only oxide formed by the metal aluminum. The precious stones ruby and sapphire are composed of corundum colored by small amounts of impurities. Fused alumina, alumina that has been melted and recrystallized, is identical in chemical and physical properties with natural corundum. It is exceeded in hardness only by diamond and by a few synthetic substances, notably carborundum, or silicon carbide. Both impure natural corundum (emery) and pure synthetic corundum (Alundum) are used as abrasives. At room temperature alumina is insoluble in all ordinary chemical reagents. Its melting point is high, slightly above 2000°C (3632°F), and so alumina is useful as a refractory, for example, for the linings of special furnaces. Alumina can be purified by fusing it with sodium carbonate. The resulting sodium aluminate is dissolved in w