Supreme Perlite Medium Grade ORGANIC 4 CU FTHorticultural grade perlite is derived from a naturally occurring, volcanic rock. When furnaced at high temperatures, perlite pops and expands up to 10 times its original volume. The resulting, lightweight particles are made up of microscopic spheres with a porous, irregular surface that stores moisture and promotes plant growth. Adding perlite to soil and growing media lightens the soil and introduces pore spaces that allow air and water to penetrate