Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly. It is similar to obsidian but is defined by the International Union of Geological Sciences as having a higher water content.[1] It is a volcanic glass; however, unlike a glass, pitchstone has an irregular hackly fracture not a conchoidal fracture. That is due to its coarser (than obsidian) crystal structure. 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.[2]
The pitchstonen ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley.[3]
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.[1] 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.
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