A form of silica rock

Vermiculite is a silica rock, kind of like mica, that expands into little hollow kernels when heated quickly at a very high temperature. It's usually added to potting mixes because it's light, it absorbs several times its weight in water, and it aids porosity.

There have been reports in the news lately that vermiculite may, in fact, be carcinogenic. So use of vermiculite in potting soils is no longer recommended.

If you want to find out more about vermiculite's chemical properties and how it's made, check out the Vermiculite Association web page at (Really.)