What is 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 called?

pearlite — 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. (Webster's 1913.)

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