These materials listed are either to help a water filter remove contaminants that make water taste bad, or that makes water poisonous to humans. The materials that remove poisonous contaminants are as follows: Carbon (charcoal), Ceramic and Diatomaceous (Siliceous) Earth/Rock.

Materials that remove contaminants that affect bad taste is mainly just sand and other type of gravel and small earth rocks

http://www.livestrong.com/article/165592-natural-materials-used-for-water-filtration/