Perlite is a naturally occurring amorphous volcanic glass. Perlite has the unique property of expanding from 4 to 20 times its original size when heated sufficiently (12 times is typical). This is known as expanding perlite.

Expanded perlite is lightweight, inert, sterile, inorganic, odorless, incombustible, asbestos-free, non-toxic, rot proof, vermin proof, and pH-neutral. When used for the filtration of liquids, expanded perlite imparts no tastes, colors, or odors and is listed in the Food Chemicals Codex (published by the National Academy of Sciences), deeming expanded perlite safe for its intended use. Expanded perlite is also found in the EPA's Safer Chemical Ingredients List (SCIL) under Processing Aids and Additives. These characteristics make perlite an extremely safe and versatile mineral.