#pitchstone

Pitchstone

During my recent research in to the Drumadoon Sill on the Isle of Arran I came across another rather interesting rock – Pitchstone. Pitchstones are a form of volcanic glass, like obsidian, they form when molten magma or lava comes in to contact with water. This rapid cooling forms tiny crystals that fuse together to resemble glass. However, pitchstone is unlike obsidian in that it has a higher water content than, up to 10%. This can be attributed to one of two things; either water which is already contained within the magma is incorporated in to the crystal structure or water is taken up by the crystals from the sea or groundwater that acts as the rapid cooler. It is thought that the difference in lustre between the two rocks is because of the higher water content, as obsidian is shiny and vitreous, pitchstone is dull and lack lustre like black tar – hence the name Pitchstone.

The fine grained texture of the rock and the fact that when it is hit a conchoidal fracture forms (see image below of a conchoidal fracture in obsidian) means that the edges of broken pitchstone are sharp; this makes the rock more than suitable to be used as a cutting tool. Early Neolithic (10,000BC – 2000BC) human tools like blades and small arrow heads have been made from pitchstone and found all over Britain, we know these tools were shaped from the rocks at Arran as a chemical analysis comparison shows identical mineral compositions. Humans spread the tools by trade through Scotland in to England and across the sea in to Ireland before they were replaced by the flint trade from Yorkshire.

POL