Pumice is a form of vocanic glass.  When it was still liquid magma it was mixed with hot gases that formed bubbles in the magma, sort of like foam.  When the magma was extruded from a volcano it cooled quickly to become a rock filled with air pockets.  Pumice is very light and its many edges are sharp. 

    It is creamy white in color but after being exposed to the air it may turn a light brown.

    Pumice is ground-up and used in cleansing powders because it is a good scouring agent.