Minerals (Category A)
Perlite is an amorphous volcanic glass (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. Expansion occurs when the glassy lava rock is heated to 1600 degrees F (871 degrees C) and the water molecules trapped in the rock turn into vapor causing the rock to expand. (This is the same principle as the water in popcorn that causes the kernel to pop when it is heated.) 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.