The Collaborative International Dictionary of English v.0.48:

Carborundum \Car`bo*run"dum\ (k[aum]r`b[-o]*r[u^]n"d[u^]m), [a
   trade name, from Carbon + corundum.]
   A beautiful crystalline compound, silicon carbide (SiC),
   consisting of carbon and silicon in combination; -- also
   called 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.
   [Webster 1913 Suppl.] carborundum cloth