rotten stone

Webster's Dictionary (1913)

Mineralogy, a soft stone, called also Tripoli (from the country from which it was formerly brought), used in all sorts of finer grinding and polishing in the arts, and for cleaning metallic substances. The name is also given to other friable siliceous stones applied to like uses.

From the entry Rotten.

Webster’s Revised Unabridged Dictionary (1913), from GCIDE 0.54