Polishing Slate, a mineral composed chiefly of silica, with a little alumina, lime, oxide of iron, and water; white, yellowish white, or yellow; and of specific gravity about half that of water. It is used for polishing glass, marble, and metals.
Polishing Slate,
Chambers's Encyclopaedia, Volume 8: Peasant to Eoumelia, p. 287
Source scan(s): p. 0296