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. [1913 Webster
" The name corundum is sometimes restricted to the non-transparent or coarser kinds. Emery is a dark-colored granular variety, usually admixed with magnetic iron ore."