Pitchstone

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.

Pitchstone ridge: An Sgùrr, Eigg, Scotland

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.[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.[1]

References
  1. Ballin, Torben Bjarke (2015). "Arran pitchstone (Scottish volcanic glass): New dating evidence". Journal of Lithic Studies. University of Edinburgh. 2 (1): 5–16. doi:10.2218/jls.v2i1.1166.
  2. al.], C.H. Emeleus ; contributors: J.D. Hudson ... [et (1997). Geology of Rum and the adjacent islands : memoir for 1:50000 geological sheet 60 (Scotland) (1. publ. ed.). London: Stationery Office. ISBN 0118845179.


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