pumice
noun[L. pumex, supposed to be from the root of spuma, foam.]
A substance frequently ejected from volcanoes, of various colors, gray, white, reddish brown or black; hard, rough and porous; specifically lighter than water, and resembling the slag produced in an iron furnace. It consists of parallel fibers, and is supposed to be asbestos decomposed by the action of fire. Pumice is of three kinds, glassy, common, and porphyritic.