Rockwool is made by melting basaltic rock and spinning it into fibers which are then compressed into slabs.
It is made to hold large quantities of water and air that aid root growth and nutrient uptake.
Good quality Rockwool should wet easily initially but not remain water soaked.
Plants grown in Rockwool will not be exposed to water stress until the Rockwool is almost completely dry which makes it a very popular medium amongst hydroponic growers.