Pitchstone is a glassy, acidic igneous rock, rather like obsidian but with a rather more waxy or resinous lustre owing to the absorption of water. The rock is glassy due to rapid quenching of the initial intrusion/extrusion.

When cooled very quickly, the magma has little or no time for crystals to grow and what results is largely a volcanic glass. Pitchstone often also exhibits flow-banding (see picture of Judd's Dyke below), well developed columnar jointing and the effects of devitrification.

A fine example on Arran would be the Corrygills Pitchstone Sill (NS 051338), which forms the large cliffs on the shore below the Clauchland Hills. A number of Judd's Dykes are composed of pitchstone (as shown on left).