Source : Webster's Revised Unabridged Dictionary (1913)

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}.