Blue John (mineral)

Bowl made from Derbyshire Blue John, on display in Castleton Visitor Centre
French candelabra crafted of Blue John, c. 1860

Blue John (also known as Derbyshire Spar) is a semi-precious mineral, a rare form of fluorite with bands of a purple-blue or yellowish colour. In the United Kingdom it is found only at Blue John Cavern and Treak Cliff Cavern at Castleton in Derbyshire. During the 19th century, it was mined for its ornamental value, and mining continues on a small scale.