Corundum
n.
The mineral alumina ( Al2O3 ), as found native in a crystalline state. Transparent varieties are used as gemstones, including sapphire , which is the fine blue variety; the oriental ruby , or red sapphire; the oriental amethyst , or purple sapphire; and adamantine spar , the hair-brown variety. It is the hardest substance found native, next to the diamond.
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