Answer: perlite

AnswerCrossword Clue
PERLITEGreenhouse soil additive
PERLITEGlassy rock
PERLITEVolcanic glass.
PERLITEVolcanic rock.
PERLITELustrous volcanic rock
PERLITEPlant growth medium
perlitevolcanic glass in which concentric fractures impart a distinctive structure resembling masses of small spheroids, used as a plant growth medium
perlitea volcanic glass