Rockwool is one of the most popular growing mediums available. Used mostly in drip hydroponic systems, rockwool is made of a combination of sand and rock melted together. The mixture is then spun into fibers that are shaped into different sizes and forms. Rockwool can hold extremely large amounts of water and air and keeps its shape very well. Rockwool does have some disadvantages: it does not break down easily when disposed of, the fibers are unhealthy and should not be inhaled, and the pH is very high.