Pumice

Pumice is vitreous and highly porous volcanic rock which causes it to have a low density (it can float on water). It is formed from fragments of viscous magma (rhyolitic, dacitic, or andesitic) which, when projected into the air by a volcano, undergo a sudden change in pressure causing degassing and the formation of bubbles separated by thin walls of volcanic glass. These light-coloured rocks are friable, and the fragments (from about 1 to 15 cm), which are angular at first, are quickly blunted.