PUMICE - Students will immediately notice that pumice is spongy or "full of holes" or vesciular. This characteristic makes pumice extremely lightweight; it even floats in water (you may wish to show this to your students). It is commonly light gray to blackish-gray in color. It is easily broken and has sharp edges. Like obsidian, pumice is volcanic glass; it thus looks glassy (especially with a magnifying glass) and lacks visible minerals.
Pumice forms during eruptions of magma containing large quantities of gasses, such as water vapor, sulfur dioxide, and carbon dioxide. The gas "froths" the magma as it erupts, forming bubbles. This is physically analogous to opening a soda can; carbon dioxide bubbles form in the drink as the can is opened. Like obsidian, the magma then cools quickly, preserving the bubble shapes. The gas often escapes, leaving numerous holes in the pumice. Pumice is used as an ornamental building stone. "Pumice rock" is also sold in beauty stores for cleaning dead skin cells from areas like feet or elbows.