Definition of "polishing-slate" []

• A slate, usually gray or yellow, composed of microscopic infusoria, found in the coal-measures of Bohemia and in Auvergne in France, and used for polishing glass, marble, and metals. (noun)
• A kind of whetstone used for sharpening or polishing the edges of tools after grinding on a revolving grindstone. (noun)

The Century Dictionary (Public Domain)