[Kartographer]

Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. 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.

http://en.wikipedia.org/wiki/Perlite

You can find it in most garden departments. Almost seems like small pieces of foam like a cooler is made of.

[Iron Munro]

Thanks