Corundum
Corundum is the crystalline mineral form of aluminum oxide (Al₂O₃), with exceptional hardness. Its gem varieties include ruby and sapphire.
Aluminum oxide: An inorganic compound of aluminum and oxygen with the formula Al₂O₃. Corundum is the naturally occurring crystalline mineral form of this compound.
Ruby: The red gem variety of corundum, colored chiefly by chromium impurities. Ruby is one of corundum’s best-known gem varieties.
Metamorphic rock: Rock transformed from pre-existing rock by heat, pressure, or chemically active fluids without melting completely. Corundum commonly forms when aluminum-rich rocks undergo metamorphism.
Diamond: A crystalline form of carbon and the hardest naturally occurring mineral on the Mohs scale. Diamond outranks corundum in scratch resistance, though the two have different crystal chemistry.
Crystal structure: The ordered, repeating arrangement of atoms in a crystalline solid. Corundum’s atomic arrangement determines its characteristic shape and physical properties.
Sapphire: The gem variety of corundum in colors other than ruby red, commonly blue. Sapphire includes blue corundum and gem-quality corundum in many other colors.
Bauxite: A mixture of aluminum-rich minerals that is the principal ore of aluminum. Corundum can form in geological settings involving aluminum-rich bauxite deposits.
Spinel: A mineral composed chiefly of magnesium aluminate, sometimes resembling ruby or sapphire in appearance. Spinel has historically been mistaken for ruby despite having a different composition and structure.
Mohs scale: A ranking of minerals by their resistance to scratching, from talc at 1 to diamond at 10. Corundum ranks 9, second only to diamond on this widely used scale.
Abrasive: A hard material used to wear down, smooth, or polish another surface. Crushed corundum can abrade materials because it resists scratching.