Pumice is a low-density volcanic rock that consists of highly vesicular, rough, textured, volcanic glass, which may or may not contain crystals. It is typically light colored. Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. This simultaneous process freezes bubbles from gases being dissolved in the lava and preserves them in a matrix, which gives pumice its highly porous texture. Pumice's highly unique process of formation yields an extremely low-density rock; in fact, pumice is the one and only rock that will not sink in water! Pumice floats and bobs along the surface where other rocks would sink to the depths. Great for understanding density and can be used in classroom experiments.