Perlite

Perlite is another light weight substance, made by heating a amorphous volcanic glass to a high temperature which pops up to a porous substance that is very good in aeration and improves drainage.  It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing.

 Caution: Perlite can cause irritation to the lungs and eyes. Protective clothing is recommended while handling perlite.