Rockwool is an artificial media made from quartz sand that has been heated above 1200° C and spun into fibers then formed into slabs, cubes, and blocks. Most production of rockwool comes from European countries and is a major substrate for the greenhouse industry, but use of rockwool is declining due to cost and lack of options for disposal of used rockwool.
The production process involving heat makes rockwool completely free of pathogens. The fibers of rockwool make it highly porous thus allowing a high air-porosity ratio in the root zone. Rockwool is inert and contains some silica that can be available to plants.
Disposal and cost of rockwool make it less than sustainable in long term production. Typically pH values of rockwool are high and are will need to be reduced to maintain optimal pH values for plant growth. Buffering capacity of rockwool is low and can be difficult to manage pH due to water retention characteristics of the fibers used.