Pumice is a type of volcanic rock formed when lava with extremely high levels of water and gases is violently ejected from a volcano. As explained by the Mineral Information Institute, when the gases escape, the rock become "frothy." Once the rock hardens, the result is a very light, buoyant material. The main use of pumice is for making lightweight construction materials such as concrete. Small "pumice stones" are commonly used at home or in beauty salons to remove dry skin and calluses from areas such as the feet.
Pumice has a chemical composition similar to that of obsidian, or volcanic glass. It has very thin, translucent bubble walls of extrusive igneous rock. Pumice stones as used in beauty salons are generally high in silica and low in iron and magnesium.
Pumice is very lightweight. This is due to the air bubbles in it, created as gasses departed during the formation process. Pumice stones vary in density, according to the thickness of the solid material between the bubbles. Average porosity is 90 percent, which is extremely high for a rock material.
Pumice stones are so lightweight they will usually float on water for a time, before eventually sinking once becoming waterlogged. Large rafts of pumice have been known to float through oceans for decades after volcanic eruptions.
Pumice stones are abrasive, which is why they are good at removing dry skin and calluses from the feet. Pumice is also used as an abrasive in polishes, pencil erasers, cosmetic exfoliants and the production of printed circuit boards.
Pumice is commonly pale in color, ranging from white, cream, blue or grey, to green-brown or black. Pumice stones found at beauty salons or in pharmacies are generally light gray in color.