pitchstone's Usage Examples:

In the south of the island there are sills and dykes of felsite, quartz-porphyry, rhyolite, trachyte and pitchstone.

Staffordshire, the diorites of Warwickshire, the phonolite of the Wolf Rock (to which he first directed attention), the pitchstones of Arran and the altered igneous rocks near the Land's End were investigated and described by him during the years1869-1879in the Quarterly Journal of the Geological Society and in the Geological Magazine.

pitchstone flake should also be noted.

In Arran there are pitchstone dikes, some of which are very completely vitreous, while others are changed to spherulitic felsites more or less silicified.

The pitchstone of the Scuir of Eigg is at its margins characterized by a dull semi-opaque matrix which seems to be the result of secondary devitrification.