Pumice

Volcanic rock

Pumice, called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles, thicker vesicle walls, and being dark-colored and denser.

From Wikipedia, under CC BY-SA. More on occurri.