Pure horticultural grade perlite. Granule size about one-eighth of an inch. Perlite is made from all-natural volcanic rock that is heated under pressure until it explodes like popcorn, leaving a white puff that is so light that it will actually float. Perlite is added to soil and soil-less potting mixes (like coconut coir) to provide additional aeration, drainage, and insulation for roots against extreme cold. Perlite helps keep soil from compacting.