Small : L 4 x 4 x 4 cm Large : L 7.7 x W 7.5 x H 6.6 cm Rockwool can hold large quantities of water and air that aid root growth and nutrient uptake in hydroponics; the fibrous nature also provides a good mechanical structure to hold the plant stable. It is used primarily for drip hydroponic systems. Rockwool is made by melting a combination of rock and sand and then spinning the mixture to make fibers which are formed into different shapes and sizes.