Pumice rocks are a type of igneous rock that goes through a number of different stages in its cycle. While there are a number of different types of igneous rock, pumice has a unique makeup as to how it forms, looks and feels.
Molten Rock
Pumice rock begins as molten rock or lava, super-heated minerals that flow out of volcanoes from deep inside the earth. This molten lava or rock will flow and pool until it cools, hardening in a specific way to make it igneous and forming pumice, granite or obsidian.
Cooling Rock
Once the lava has cooled, it will form rock. This rock cools in different ways; pumice rock contains a number of tiny air pockets within it, giving a porous texture and a light feel. Some pumice stone that is incredibly porous will actually float in water. However, this also means pumice stone can be broken down easily, as it is less solid then denser igneous rock.
Metamorphic
Over time, with intense heat and pressure, pumice stone will break down and crumble and scatter about, or become more compact, forming a new type of rock in the rock cycle. As pumice rock compacts on its air pockets, it becomes denser and heavier, becoming a different type of rock altogether, combining with other rocks to become metamorphic rock. These rocks include schist, slate and gneiss.