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.