perlite

A natural volcanic glass similar to obsidian but having distinctive concentric cracks and a relatively high water content. In a fluffy heat-expanded form perlite is used as a lightweight aggregate, in fire-resistant insulation, and in soil for potted plants.

A peculiar form of certain vitreous rocks, such as obsidian and pitch-stone, the mass of which sometimes assumes the form of enamel-like globules.

See pearlite.

Same as pearlite.

An amorphous volcanic glass formed by the hydration of obsidian.