Supreme Perlite Coarse Horticultural Grade Perlite consists of a combination of large, medium, and fine perlite particles designed to provide balanced moisture-holding and drainage when mixed with soil or other growing media substrates such as peat moss, coco coir, and compost. Perlite is a 100% natural, heat-expanded siliceous volcanic rock. It is used to loosen and lighten heavy clay soils or as a medium for seed starting and root cuttings. Perlite is sterile, ultra-lightweight, non-toxic, durable, incombustible, asbestos-free, and virtually pH-neutral.