Chapman, Carleton A. Department of Geology, University of Illinois, Urbana, Illinois.
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A natural glass with dull or pitchy luster and generally brown, green, or gray color. It is extremely rich in microscopic, embryonic crystal growths (crystallites) which may cause its dull appearance. The water content of pitchstone is high and generally ranges from 4 to 10% by weight. Only a small proportion of this is primary; most is believed to have been absorbed from the surrounding regions after the glass developed. Pitchstone is formed by rapid cooling of molten rock material (lava or magma) and occurs most commonly as small dikes or as marginal portions of larger dikes. See also: Igneous rocks; Volcanic glass
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