Mother Earth? Perlite is naturally occurring siliceous rock that expands up to 20 times its original volume when heated to 1600? F. This kilning process is very similar to popping popcorn and produces a growing media that is sterile and has a neutral pH of 7.0. The exceptional physical properties of this perlite make it tops in the perlite world for adding porosity to soil and soilless mixes. This perlite has numerous hydroponic applications as well. The #4 is the largest perlite on the market and averages a whopping size of around 1 in. The #3 measures around 1/2 in and the Coarse is about 1/8 to 1/4 in.