alumina (aluminium oxide, Al₂O₃) Mineral used as an abrasive, electrical insulator and furnace lining. Other forms of alumina include corundum, two impure varieties of which are the gemstones sapphire and ruby.

a·lu·mi·na / əˈloōmənə/ • n. aluminum oxide, Al₂O₃, a white solid that is a major constituent of many rocks, esp. clays, and is found crystallized as corundum, sapphire, and other minerals.