WHITTEMORE Perlite Supercoarse 4 CFPure horticultural grade perlite. Grade 1, Fine Coarseness (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