Porcellanite

At Tievebulliagh, Northern Ireland, porcellanite is a tough contact metamorphosed hornfels formed from a lateritic soil horizon within a basaltic intrusive/extrusive sequence. The rock is black to dark grey in colour.

Tievebulliagh is the site of a Neolithic axe or stone tool quarry, and there is another quarry on Rathlin Island.^[2] It is likely that roughouts or roughly-shaped prehistoric tools (called celts) were chipped on site before transportation both within Ireland and over the Irish Sea to Britain.^{[citation needed]} It is also likely that the final polish would have been performed near the site of use in cutting vegetation and trees.^{[citation needed]} It was commonly polished on grooved blocks of hard sandstone.^{[citation needed]}