Perlite

Perlite is made by heating a certain type of volcanic rock to a high temperature whereupon it puffs up (a bit like popcorn) into a large and absorbent granule. It holds a lot of water but still drains very well. Some growers claim to have had success using it on it's own as a hydroponic medium, but it is much more often used as an additive to soil or coco to improve drainage.