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  Pitchstone is a dull black glassy volcanic rock formed when lava cools swiftly. It is similar to but coarser than obsidian.

The ridge of An Sgurr on the Isle of Eigg was originally formed as a lava flow in a valley. Pitchstone is very resistant to erosion.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pitchstone". A list of authors is available in Wikipedia.
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