\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.