Aluminium oxide is the main component of bauxite, the principal ore of aluminium. Industrially, bauxite is purified to aluminium oxide via the Bayer process, and then converted to aluminium metal in the Hall_Heroult process.
The gems ruby and sapphire are mostly aluminium oxide, given their characteristic colors by trace impurities.
Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin layer of aluminium oxide quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising.
Aluminium oxide is an excellent thermal and electrical insulator. In its crystalline form, called corundum, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.
Powdered aluminium oxide is frequently used as a medium for chromatography.
For example, when thermite (a mixture of powdered iron oxide and aluminum) is heated, the aluminum rapidly removes the oxygen from the iron; the heat of the reaction is sufficient to melt the iron.
Aluminum is never found as a free metal; commonly as aluminum silicate or as a silicate of aluminum mixed with other metals such as sodium, potassium, iron, calcium, and magnesium.
Bauxite, an impure hydrated aluminumoxide, is the commercial source of aluminum and its compounds.
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