Aluminum is a strong, light metal, especially corrosion-resistant because of its tightly adhering oxide coating of . If aluminum metal lacked this oxide film, it would be sufficiently reactive to break down warm water as magnesium does:



Crystalline aluminum oxide or corundum, , is used as an abrasive. Rubies are crystals with a few of the ions replaced by the transition-metal ion , which makes rubies deep red. Blue sapphires have a small amount of ions instead. Artificial rubies now are made in commercial quantities for use as bearings in watches and other precision instruments. Pure aluminum is obtained by electrolysis of bauxite, a hydrate of . (Hydrates have definite amounts of water as part of a crystal structure, as in natural borax above.)