corundum

noun

1. very hard mineral used as an abrasive

   noun.substance

   Synonyms corundom

   Source: WordNet 3.0

2. The earth alumina, as found native in a crystalline state, including <i>sapphire</i>, which is the fine blue variety; the <i>oriental ruby</i>, or red sapphire; the <i>oriental amethyst</i>, or purple sapphire; and <i>adamantine spar</i>, the hair-brown variety. It is the hardest substance found native, next to the diamond.

   noun

   Source: webster1913