Information from the Getty AAT
pumice (volcanic rock)
A pale gray, porous variety of the volcanic stone rhyolite; it is composed of potassium aluminum silicate with small amounts of iron and alkalis. Pumice is used as an abrasive for polishing jewelry, cleaning metals, and smoothing vellum and parchment. In its solid form it is used as an abrasive and aggregate and in its powdered form it is used as a polish and abrasive.