Pumice is porous volcanic rock characterised by its network pores and bubbles held in a light coloured volcanic glass. Pumice has a very low density and can even float on water. All types of magma (basalt, andesite, dacite, and rhyolite) can form pumice. Pumice is formed when liquid gases dissolved in the magma suddenly expands to form a froth or foam. Pumice is this solidified foam.