Rockwool is a mineral made of a basalt and silica compound. It is used as a grow medium for hydroponic growing because it is cheap, easy to use and can easily retain water.

Learn all about growing with rockwool in this article:

• Hydroponic Marijuana Growing Indoors

More grow medium definitions:

Grow medium | Soil | Seedling & Cutting soil | Hydroponic | Clay pellets | Coconut fiber | Perlite | Vermiculite | Aeroponic