Pumice Samples 25 Pack - Your class will erupt with excitement as each student explores and observes the product of an actual volcano. Pumice is a lightweight igneous rock, formed when lava cools quickly above Earth's surface and traps gases before the molten material solidifies. As a result, it contains so many air holes that it floats in water! These are a naturally occurring item. Size and shape are dependent on availability and will vary. Note: Pumice is a naturally occurring material. It is normal for it to be soft and dusty. This will clean off with use.