Frosterley Marble

Frosterley Marble is a black, bituminous coraliferous limestone containing fossil crinoids of the Lower Carboniferous (Mississippian ), some 325 million years ago.

[1][2]: 40  It outcrops in Weardale, County Durham, England, including near the village of Frosterley whence it is named.

Unlike a true marble, it is not a metamorphic rock, but is so-called because it can take a fine polish.

Concentrations of such corals as Dibunophyllum bipartitum and of brachiopod remains contribute to its attractiveness when sections are polished.

[3][2]: 40 It is cut and polished for use as ornate stone, and was much desired for church decoration, particularly during the Middle Ages.