Perlite's an amorphous volcanic glass that's 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's an industrial mineral and a commercial product useful for its light weight after processing. Properties When it reaches temperatures of 850–900 °C, perlite softens (since it's a glass). Water trapped in the structure of the material vapourises and escapes and th… (More on Perlite)