Pumice

Pumice, igneous rock having a spongy or frothy texture, and composed largely of glass. It is frequently made up of parallel fibres or threads, with intervening spaces to form a delicate structure. Pumice is produced by the expansion of the occluded, or internal, gases of lavas when they reach the surface of the Earth. It is most abundantly developed in rhyolite lavas, because these are usually very viscous. It occurs, for example, around the volcanoes of Mount Etna, Vesuvius, and Stromboli, Italy. It is still sometimes used, as “pumice stone”, to abrade the skin when bathing.