The name perlite (also spelled pearlite) comes from the French word perle which means pearl, in reference to the “pearly” luster of classic perlite.

Perlite is a type of volcanic glass that expands and becomes porous when it is heated. When heated, it can expand to as much as twenty times its original volume. This expansion is the result of heated water: when the glassy lava rock is heated to 900-1000 ^oC, the water molecules trapped in the rock turn into vapor which causes the rock to expand. (This is the same principle as the water in pop corn 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.

Volcanic glass forms when molten rock (lava) pours out of a volcano and cools very, very quickly. Because is cools so quickly, there is no time for crystals to form or for water to escape. Instead, the lava hardens immediately into this glass-like material containing 2-5% water. It is a silicate rock, which means that it has a high percentage of silica (Si).

Perlite is known in industry in two forms. Crude perlite is prepared by the crushing and screening of perlite into various size fractions. expanded perlite is perlite after it has been heated.