Perlite is a volcanic glass that is heated to 1,600 degrees F whereupon it pops much like popcorn and expands to 12 times its former size, resulting in an incredibly lightweight material. The super heated perlite comprises of tiny air compartments.

 Under a microscope, perlite is revealed as being covered with many tiny cells that absorb moisture on the exterior of the particle, not inside, which makes it particularly useful in facilitating moisture to plant roots.