Torben Bjarke Ballin, Honorary Research Fellow at the University of Bradford and member of the IPG has published a paper in the Bulletin of the International Association of Obsidian Studies describing the results of his study of pitchstone. His project, begun in 2004, led to the production of a database of 5,542 pieces of archaeologically worked pitchstone derived from around 350 sites. A further 14,707 artefacts are noted on the database but were not examined in person.
Torben’s work has shown that pitchstone was in use from the Mesolithic to the Early Bronze Age on Arran, but there is no evidence for its use in mainland Scotland during the Mesolithic and little evidence for its use off Arran during the Bronze Age.
Torben’s paper can be found here.