Derbyshire Blue John

Blue John (also known as Derbyshire Spar) 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 mined for its ornamental value, and mining continues on a small scale. Deposits of fluorite have been recently found in China with colouring and banding similar to the classic Blue John stone.

The most common explanation for the name is that it derives from the French bleu-jaune, meaning 'blue-yellow'. The story goes that Blue John was exported to France where it was used by ormolu workers during the reign of Louis XVI (1774–91). However, there is no archival record of any Blue John being exported to France, and the early ormolu ornaments which use Blue John were being manufactured by Matthew Boulton of Birmingham in the 1760s.

An alternative origin of the name derives from an old miners' name for the zinc ore sphalerite which they called "Black Jack". Thus the unique blue stone mined in these caverns could easily have become known as "Blue John".

Another derivation comes 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, and derives from the Cornish language word bleujenn, in Old Cornish blodon, a flower, bloom or blossom.

In the United Kingdom, the blue, banded, fluorite known as Blue John is found only under the triangular hill known as Treak Cliff, just outside the town of Castleton. Today the veins are mined only in Blue John Cavern and the nearby Treak Cliff Cavern, although the abandoned Old Tor Mine on the north side of Winnats Pass was also, at one time, a source for the mineral.

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