The Collaborative International Dictionary of English v.0.48:

Corundum \Co*run"dum\ (k[-o]*r[u^]n"d[u^]m), n.; pl. Corundums
   (k[-o]*r[u^]n"d[u^]mz). [Also corindon.] [From Hind. kurand
   corundum stone.] (Min.)
   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 +PJC]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

corundum
    n 1: very hard mineral used as an abrasive [syn: corundom,
         corundum]