Corundum
A mineral almost equal to the sapphire in hardness, and for that reason it is employed, like emery, for polishing metals and hard stones. It is pulverized, mixed with glue, and other gum compositions, and moulded into all forms of wheels for grinding and polishing every variety of articles produced in the metal industry.
The colors of corundum are various, including green, blue, red, inclining to gray, of a shining lustre, translucent or opaque; specific gravity, 4; fusible only by the compound blowpipe. The composition of corundum is: crystalline alumina 85.5, silex 7, oxide of iron 14. It is found in the East Indies, China, the United States, and other places. The Chinese varieties are called adamantine-spar. See EMERY; PRECIOUS STONES.