a light glass formed on the surface of some lavas; used as an abrasive
A light, porous type of pyroclastic igneous rock, formed during explosive volcanic eruptions when liquid lava is ejected into water or air as a froth containing masses of gas bubbles. As the lava solidifies, the bubbles are frozen into the rock.
rub with pumice, in order to clean or to smoothen
To abrade or roughen with pumice.