Liddicoat, Richard T., Jr. Gemological Institute of America, Los Angeles, California.
Last reviewed:January 2020
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Any gem variety of the mineral corundum (Al2O3), except those called ruby because of their medium- to dark-red color. Sapphire has a hardness of 9 (Mohs scale), a specific gravity near 4.00, and refractive indices of 1.76–1.77.
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