rottenstone

noun

A soft decomposed limestone, used in powder form as a polishing material.

To polish with rottenstone.

An argillaceous or silicious limestone which by weathering has become soft and friable, the calcareous part having been wholly or in part removed.

A soft, friable limestone, used in polishes.

A weathered and decomposed siliceous limestone; in powdered form it is used in polishing.