Perlite forms when a body of rhyolite
, for one reason or another, has a high water content. Perlite often has a perlitic texture, typified by concentric fractures around closely spaced centers and a light color with a bit of pearlescent shine to it. It tends to be lightweight and strong, an easy-to-use building material. Even more useful is what happens when perlite is roasted at around 900°C, just to its softening point it expands like popcorn into a fluffy white material, a mineral Styrofoam.
Expanded perlite is used as insulation, in lightweight concrete, as an additive in soil (such as an ingredient in potting mix), and in many industrial roles where any combination of toughness, chemical resistance, light weight, abrasiveness and insulation is needed.
See more pictures of perlite and its cousins in the gallery of volcanic rocks.