Vermiculite is a popular hydroponic media favored for its affordability.
It is made from a natural mineral that expands with the application of heat. The expansion process is called exfoliation which takes place in purpose built commercial furnaces.
Vermiculite is very lightweight and sterile.
It has excellent water retention and capillary action properties, which allows it to be fully hydrated simply by applying moisture from above or below.
When used in hydroponics, vermiculite is often mixed with perlite if the media becomes too water logged. This improves aeration and drainage.