Source : Webster's Revised Unabridged Dictionary (1913)

Carborundum \Car`bo*run"dum\, [Carbon + corundum.]
   A beautiful crystalline compound, {SiC}, consisting of carbon
   and silicon in combination; carbon silicide. It is made by
   heating carbon and sand together in an electric furnace. The
   commercial article is dark-colored and iridescent. It is
   harder than emery, and is used as an abrasive.

Source : WordNet®

carborundum
     n : an abrasive composed of silicon carbide crystals