Rotten stone, sometimes spelled as rottenstone, also known as tripoli, is fine powdered porous rock used as a polishing abrasive for metalsmithing and in woodworking. It is usually weathered limestone mixed with diatomaceous, amorphous, or crystalline silica. It has similar applications to pumice, but it is generally sold as a finer powder and used for a more glossy polish after an initial treatment with coarser pumice powder. Tripoli particles are rounded rather than sharp, making it a milder abrasive. Wikipedia, Rotten Stone

Ramsay
A very important thing in the making of a violin, after a good form, a right balancing of part against part, and all of wood in skillful condition, is the violin varnish. Composition:-

Linseed oil boiled,... ... ... 1 part.
Isinglass, ... ... ... ... 1/2 part.
Turpentine, ... ... ... ... Quantum suf.

Give two coats with this, then rub down with fine sandpaper. Then, best copal varnish, one coat. Finish then with boiled linseed oil, thickened with sifted 'rotten stone.' This gives a fine, smooth, and dull surface. Ramsay's violins are of surpassing tone; and he considered the varnish an important element in violin-making. [Scientific Basis and Build of Music, page 85]