pumice

A porous, frothlike volcanic glass which did not crystallize due to rapid cooling and frothed with the sudden release of dissolved gases. It is light in color and is lightweight and buoyant enough to float on water. In its solid form it is used as an abrasive and aggregate and in its powdered form it is used as a polish and abrasive. It often has the composition of rhyolite. Obsidian changes into pumice when it is melted.