Columbia Encyclopedia World Factbook Columbia Gazetteer American Heritage Coll. Usually found at the surface of a lava flow, it is colorless or light gray and has the general appearance of a rock froth. The viscosity of the lava, the quantity of water vapor and gas, and the rate of cooling together determine the fineness of the vesicular substance. Large amounts of gas result in a finer-grained variety known as pumicite. The chemical composition is that of granite.
Pumice is used chiefly as an abrasive and is included in many scouring preparations.
Ground pumice is also used in finishing furniture. Deposits are found in volcanic areas throughout the world. Because of its air chambers, pumice has a very low density and has been observed blowing off volcanic islands in strong winds. It usually floats and can be carried great distances by ocean currents. The Columbia Encyclopedia, Sixth Edition.