pumice

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.