Blue John

Derbyshire Blue John (also known as Derbyshire Spar, or simply Blue John) is a semi-precious mineral, a form of fluorite with bands of a 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 extensively mined for its ornamental value, and made into wide variety of ornamental items ranging from knife-handles to chalices. Mining continues today but only on a small scale.

There are various explanations as to how the name came about. The commonest is that it derives from the French bleu-jaune, meaning 'blue-yellow'. Others believe that it came from the Cornish miners who began working the Derbyshire lead mines in the 1740s. The name Blue John is used for several rocks in Cornwall, including fluorspars, deriving from the Cornish language word bleujenn, a flower, bloom or blossom.

The Terror of Blue John Gap is a short story by Sir Arthur Conan Doyle, which appeared in The Strand Magazine of 1910 and describes the experiences of a doctor who, while recuperating from tuberculosis on a Derbyshire farm, investigates mysterious goings-on in a cavern mined for Blue John.

Blue John is a very beautiful mineral and deserves inspection through the looking glass.