Perlite is a granular-type loose-fill insulation quarried mainly in the western United States. It is a naturally occurring silicous rock. Perlite is different from other volcanic glasses because when the crushed ore is heated to a suitable point in its softening range, it expands from 4 to 20 times its original volume. This expansion is attributed to the presence of water, between 2 and 6 percent, in the crude perlite rock. When the rock is quickly heated to above 1800°F, the material pops in a manner similar to popcorn as the trapped water vaporizes to form microscopic cells, or voids, in the heat-softened glass. After expansion, an air blast separates and grades the particles according to size. This expansion process accounts for the lightweight quality of expanded perlite. While the crude rock may range from transparent light gray to glossy black, the color of expanded perlite ranges from snowy white to grayish white.