Rotten stone is the common name for a stone also known as Tripoli (yes, named after the country it is brought from). It's a soft stone, and used in all sorts of polishing (like metals) and grinding. The name is a generic name, and there is also a different stone that is called the same thing because it's used in a similar way.

Infromation from: http://dictionary.reference.com/search?q=rotten%20stone Also see Webster 1913's definition Tripoli