Porcellanite or porcelanite, is a hard, dense rock somewhat similar in appearance to unglazed porcelain. It is often an impure variety of chert containing clay and calcareous matter.^[1] Porcellanite has been found, for example, in Northern Ireland, Poland and the Czech Republic. Porcellanite is also commonly found in the Northern Territory of Australia. There, it comes in a variety of colours, primarily white, yellow, red and purple.

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 (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]}

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