Perlite

Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral, suitable "as ceramic flux to lower the sintering temperature", and a commercial product useful for its low density after processing.

It is also used as a soil conditioner in horticulture.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0

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