Pitchstone
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. Pitchstone has a resinous lustre, or silky in some cases, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is an extrusive rock that is very resistant to erosion.[1]
The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley.[2]
Pitchstone from the Isle of Arran was used as the raw material for making various items from the Mesolithic through the Neolithic to the Early Bronze Age.[3] Mesolithic use appears to have been limited to the Isle of Arran itself, while in later periods the material or items made from it were transported around Britain.[3]
References
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