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Expanded volcanic glass, horticultural aggregate, soil aerator, lightweight soil amendment, volcanic glass granules
Amendment of sand or volcanic glass expanded by heat, which aerates the soil or growth medium.
Merriam-Webster Online Dictionary
perlite (noun) volcanic glass that has a concentric structure, appears as if composed of concretions, is usually grayish and sometimes spherulitic, and when heated expands to form a lightweight aggregate used especially in concrete and plaster and as a medium for potting plants
Perlite (Wikipedia)
For the two-phased structure in steel, see pearlite.
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.

Perlite (Wiktionary)
English
Etymology
From French perle (“pearl”) + -ite.
Pronunciation
- Homophone: pearlite
Noun
perlite (plural perlites)
- (mineralogy) An amorphous volcanic glass formed by the hydration of obsidian.
- The lightweight insulating material and aggregate resulting from expanding perlite glass by heat.
Translations
Anagrams
- Peltier, peitrel, rep-tile, reptile, triple-E