Mother Earth® Perlite is naturally occurring siliceous rock that when heated to 1600° F expands up to 20 times its original volume. This kilning process is very similar to popping popcorn and produces a growing medium that is sterile and has a neutral pH of 7.0. The exceptional physical properties of this giant perlite make it tops in the perlite world for adding porosity to soil and soilless mixes. This giant perlite has numerous hydroponic applications as well. The #4 averages a whopping size of around 1 in and the #3 measures around 1/2 in.