noun Petrography.
a volcanic glass in which concentric fractures impart a distinctive structure resembling masses of small spheroids, used as a plant growth medium.
Also, pearlite.

1825–35; < French; see pearl1, -ite1

perlitic [pur-lit-ik] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
perlite or pearlite (ˈpɜːlaɪt)
a variety of obsidian consisting of masses of small pearly globules: used as a filler, insulator, and soil conditioner
[C19: from French, from perlepearl1]
pearlite or pearlite
[C19: from French, from perlepearl1]
perlitic or pearlite
pearlitic or pearlite

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
Concrete blocks are also sometimes filled with vermiculite or perlite pellets.
Some less common types of loose-fill insulation include polystyrene beads and
  vermiculite and perlite.
Older homes may have granular vermiculite or perlite loose-fill insulation that
  is no longer used.
Processed perlite is used as loose fill insulation material or bound into slabs.
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