Perlite is an amorphous volcanic glass lava rock (SiO2) that has relatively high water content, typically formed by the hydration of obsidian. Perlite has the unusual characteristic of expanding and becoming porous when it is heated. It can expand to as much as twenty times its original volume. Expanded when is heated to 1600 degrees F (871 degrees C) and the water molecules trapped in the mineral turn into vapor causing it to expand. (simple like grain to popcorn.) Before it is expanded, perlite is commonly gray, but can also be green, brown, blue or red. After it has been heated, perlite is typically light gray to white. .
It is an industrial mineral and a commercial product useful for its light weight after processing.