Blue John, from Witches Miller Vein, Treak Cliff Cavern, Castleton, UK (REF:BJC4)

Blue John is a semi-precious mineral, a form of Fluorite with bands of purple-blue or yellowish colour. In the UK 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 valve, and mining continues on a small scale.

The name Blue John is believed to come from French 'bleu-yellow', and is also referred to as Derbyshire Blue John and Derbyshire Spar.