Perlite is a volcanic glass that is heated to about 1000 ºC whereupon it pops much like popcorn and expands to 13 times its former size, resulting in an incredibly lightweight material.
Perlite is commonly used by gardeners in soil and soil-less growing mixes. It greatly improves aeration and helps with moisture retention. It keeps your growing medium loose and well-drained. It is naturally sterile, free of disease, insects, and seeds, and is also pH neutral.