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
Expanded volcanic glass, horticultural aggregate, soil aerator, lightweight soil amendment, volcanic glass granules
