Corundum

Corundum: Aluminum Oxide (Al₂O₃) that crystallizes in bipyramidal prisms (Trigonal System); colors include colorless, blue (sapphire), green, yellow, orange-pink (padparadscha), pink, and purplish-red (ruby); density 3.90–4.10 g/cm³; Mohs hardness 9; refractive indices 1.757–1.770 and 1.765–1.779.

corindone
(Photo: Géry Parent/Flickr, License CC BY-ND 2.0)

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