Artificial intelligent assistant

pearl-stone

ˈpearl-stone
  The same as perlite.

1800 Henry Epit. Chem. (1808) 364 The same skilful analyst has found potash in Hungarian pearl-stone. 1852 Th. Ross Humboldt's Trav. I. ii. 102, I consider even the pearlstone as an unvitrified obsidian.

Oxford English Dictionary

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