The Collaborative International Dictionary
Pearlstone
Pearlite \Pearl"ite\, Pearlstone \Pearl"stone`\, n. (Min.) A glassy volcanic rock of a grayish color and pearly luster, often having a spherulitic concretionary structure due to the curved cracks produced by contraction in cooling. See Illust. under Perlitic.
Wiktionary
pearlstone
n. (context mineral English) perlite