|There is more information available on this subject at Pumice on the English Wikipedia.|
Pumice is a volcanic rock that is solidified lava composed of highly micro-vesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. In layman's terms it is essentially a variety of rock formed by the cooling of lava with a significant number of air bubbles, a characteristic which is responsible for its porous nature since when the lava cools, these air bubbles create tiny hollow pockets within the stone.
It is commonly, but not exclusively composed of silicic or felsic materials or otherwise intermediate in composition (e.g. rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but occurrences of basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream or gray, but can be green brown or black. It forms when gases exsolving from viscous magma nucleate bubbles which cannot readily decouple from the viscous magma prior to chilling to glass. Pumice is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. Pumice has an average porosity of 90%, and initially floats on water.