Emery

Webster's Dictionary

(n.) Corundum in the form of grains or powder, used in the arts for grinding and polishing hard substances. Native emery is mixed with more or less magnetic iron. See the Note under Corundum.

The Nuttall Encyclopedia

A dull, blue-black mineral, allied in composition to the sapphire, but containing a varying quantity of iron oxide; is found in large masses; is exceedingly hard, and largely used in polishing metals, plate-glass, and precious stones.