Rockwool

Used in hydroponics, rockwool is a mixture of igneous rock and limestone that is heated and spun into mats. Even when saturated rockwool holds about 15% air space. Rockwool provides an excellent, well aerated rooting environment that is a far superior medium for reliable and uniform seedling establishment compared to other media such as cheesecloth, blue blotter paper, brown germination paper, filter paper, fiberglass matting, agar, and soil or vermiculite filled straws.