Pumice is a very light stone that comes from volcanic rock. Formed from lava, pumice is porous, or full of tiny holes. It may sound exotic, but you can buy one at the drug store and scrub your feet with it.
The pumice found by a geologist or displayed in a museum exhibit about volcanoes is the same material as the pumice stones that you'd use to smooth the callouses off your feet. Pumice is also useful for "aging" denim (or making stone-washed jeans), and it's included in objects like pencil erasers and cinder blocks. The word comes from the Latin pumex, which shares a root with "foam."