Rockwool is a popular hydroponic growing medium made from spun basalt rock fibers. It is commonly used to start seeds or root cuttings and provides a sterile, pH-neutral and porous environment for roots to grow. Rockwool cubes are pre-soaked in pH-balanced water and can be placed into net pots or trays for plants to grow in. It is important to properly prepare and handle rockwool to prevent any potential health hazards associated with the fibers. Proper disposal of used rockwool is also important to prevent environmental contamination.