Pumice, a porous, glassy form of lava formed during volcanic explosions. Pumice contains thousands of tiny holes formed by the expansion of gases trapped in the lava as it hardens. Because of the holes, pumice has a low density and will float in water. Pumice is light gray in color and usually contains a large percentage of silica.
Pumice in powdered form is widely used as an abrasive and as a polishing agent for glass, marble, and other substances. Some tooth powders and cleaning agents also contain pumice. Ballast under railway tracks is sometimes made up of pieces of pumice. Pumice is also used in building materials, primarily as an aggregate in concrete. Pumice concrete is lighter in weight and a better insulator than ordinary concrete.