Rockwool is made from taking igneous rock, melting it until it becomes a liquid, then spinning it out like cotton candy. The end result is something with a lot of little nooks and crannies for your roots to hold onto. The drawback is some of it will break into a fine powder when you first set it up. This powder is not only dusty and cumbersome to clean up, but also bad for breathing. Wear a mask when handling it. Silica Rock Silica rock is a gritty, silicon-based rock that is dried in a kiln to harden it and change its physical properties. It has the advantage of retaining water well,but the disadvantage of constantly shedding dust until it’s nothing but a fine powder. It needs a constant, strong rinsing, so is best used with more advanced systems.