Pitchstone

Rarity: Common

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Science and technology

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What Wikipedia says

Pitchstone is a dark coloured, glassy volcanic rock formed when felsic lava or magma cools quickly. Since it is a volcanic glass, pitchstone may have a conchoidal fracture. Pitchstones may also contain phenocrysts, in which case it is a form of vitrophyre.

Excerpt from the Wikipedia article “Pitchstone”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Lysippos, CC BY-SA 3.0, via Wikimedia Commons.