pitchstone

[pich-stohn]
noun
a glassy volcanic rock having a resinous luster and resembling hardened pitch.

Origin:
1775–85; translation of German Pechstein. See pitch2, stone

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pitchstone (ˈpɪtʃˌstəʊn)
 
n
a dark glassy acid volcanic rock similar in composition to granite, usually intruded as dykes, sills, etc
 
[C18: translation of German Pechstein]

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Encyclopedia Britannica
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pitchstone

a volcanic glass with a conchoidal fracture (like glass), a resinous lustre, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is formed by the rapid cooling of viscous lava or magma.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The rate of cooling is too rapid to permit crystallization, which results in the formation of obsidian and pitchstone.
Some of the later flows are also formed of obsidian and pitchstone through rapid cooling.
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