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.
The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley.
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. 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.
References
Troll, Valentin R.; Emeleus, C. Henry; Nicoll, Graeme R.; Mattsson, Tobias; Ellam, Robert M.; Donaldson, Colin H.; Harris, Chris (2019-01-24). "A large explosive silicic eruption in the British Palaeogene Igneous Province". Scientific Reports. 9 (1): 494. Bibcode:2019NatSR...9..494T. doi:10.1038/s41598-018-35855-w. ISSN 2045-2322. PMC 6345756. PMID 30679443. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6345756 ↩
Emeleus, C.H. (1997). Geology of Rum and the adjacent islands : memoir for 1:50000 geological sheet 60 (Scotland). London: Stationery Office. ISBN 0118845179. 0118845179 ↩
Ballin, Torben Bjarke (2015). "Arran pitchstone (Scottish volcanic glass): New dating evidence". Journal of Lithic Studies. 2 (1). University of Edinburgh: 5–16. doi:10.2218/jls.v2i1.1166. hdl:10454/9048. https://doi.org/10.2218%2Fjls.v2i1.1166 ↩
Ballin, Torben Bjarke (2015). "Arran pitchstone (Scottish volcanic glass): New dating evidence". Journal of Lithic Studies. 2 (1). University of Edinburgh: 5–16. doi:10.2218/jls.v2i1.1166. hdl:10454/9048. https://doi.org/10.2218%2Fjls.v2i1.1166 ↩