Rottenstone is an ultra-fine, ash brown powdered abrasive made from finely grounded limestone. It has been used for centuries to rub out shellacked, lacquered, or varnished surfaces. It's typically used wet with either paraffin oil or water as a lubricant. With water, the abrasive cuts faster but leaves a dull sheen; oil, on the other hand, lubricates better: the cutting is slower, but the sheen is a little higher. A common practice is to rub first with pumice stone then follow with rottenstone, which is the finer abrasive.
Although rottenstone is non-toxic, use a filter mask to avoid breathing in dust particles. 1 lb. container with sprinkle top.