Alumina, comprising aluminum oxide, Al2O3, is a mineral used as an abrasive, electrical insulator, and furnace lining. The impure hydrated form of alumina, bauxite, is the chief ore of aluminum. Other forms of alumina include corundum, two impure varieties of which are the gemstones ruby and sapphire.
Related category• GEOLOGY AND PLANETARY SCIENCE
Source: U.S. Geological Survey
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