Porcellanite

Porcellanite or porcelanite, is a hard, dense rock somewhat similar in appearance to unglazed porcelain.

At Tievebulliagh, Northern Ireland, porcellanite is a tough contact metamorphosed hornfels formed from a lateritic soil horizon within a basaltic intrusive/extrusive sequence.

[2] It is likely that roughouts or roughly-shaped prehistoric tools (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.

Porcellanite from the Czech Republic
Porcellanite layer is the black rock above the hammer, and below the brown layer higher up the slope at Tievebulliagh
Porcellanite worked into Neolithic axes, Northern Ireland