pumice, volcanic glass formed by the solidification of lava that is permeated with gas bubbles. 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. Coarser-grained rock, with fewer and larger air spaces, is called scoria; it is usually associated with dark-colored igneous rocks of diorite or gabbro composition. 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.

Very porous, frothlike volcanic glass that has long been used as an abrasive in cleaning, polishing, and scouring compounds. It is also used in precast masonry units, poured concrete, insulation and acoustic tile, and plaster. Pumice is igneous rock that cooled so rapidly there was no time for it to crystallize. When it solidified, the vapours dissolved in it were suddenly released, and the whole mass swelled up into a froth that immediately consolidated. Any type of lava may become pumiceous under favourable conditions.

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