Source : Webster's Revised Unabridged Dictionary (1913)

Corundum \Co*run"dum\ (k?-r?n"d?m), n.; pl. {Corundums} (-d?mz).
   [Also corindon.] [From Hind. kurand corundum stone.] (Min.)
   The earth alumina, as found native in a crystalline state,
   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.

   Note: 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.