Safer® Hydro FAQ: What Is Rock Wool?
No sheep are needed to make this hydroponic planting media. Rather, rock wool (sometimes sold as stone wool) is made by heating basalt rock and chalk to extremely high temperatures and spinning them in chambers that turn them into fibers.
The mass of fibers are compressed into mats and cut into slabs and cubes for use in indoor gardens.
Rock wool is popular with hydroponic growers because it is purified by heat, pH-neutral, lightweight, drains well and can be reused.