Porcellanite

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]

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At Tievebulliagh, Northern Ireland porcellanite is a tough contact metamorphosed hornfels formed from a lateritic soil horizon within a basaltic intrusive/extrusive sequence. It is the site of a Neolithic axe or stone tool quarry.^[2]

References

1. ^ Porcellanite, Encyclopædia Britannica Premium Service. [1]
2. ^ http://www.habitas.org.uk/escr/summary.asp?item=14