Pumice is categorized as an extrusive igneous rock. Extrusive rock formations are made when molten lava flows from the interior of the Earth and solidifies on the surface.
Pumice is a light, porous stone with a rough surface. It is often used to exfoliate and soften rough skin on areas such as the soles of the feet and elbows. Pumice gets its appearance and texture from the way it is formed. Because it is a type of extrusive rock, pumice is formed from hot lava, or magma.
As the lava cools when it hits the air, it hardens into rock. The type of stone formed depends on the amount of gas, as well as the number of crystals that grow. For example, if the lava fails to form any crystals, obsidian is formed. When the lava releases a large number of volatile components as free gas, light-weight stones, like pumice, are formed. As the magma cools, the gas creates bubble-shaped cavities of different sizes. These cavities are what gives the stone its buoyancy and rough texture. In addition to pumice, other examples of this kind of rock are scoria and vesicular basalt.
An easy way to remember the difference between extrusive and intrusive rocks is associating the lava flow with the name. In extrusive rocks, the lava flows to the exterior of the Earth's crust. Intrusive rocks are formed by lava on the interior.