Blue John Cavern

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Entrance to the Blue John cavern

The Blue John Cavern is one of the four show caves in Castleton, Derbyshire, England.

The semi-precious mineral "Blue John" or "Derbyshire Spar" is mined from this cave. Although the cavern works as a show cave, the mineral is still worked here during the winter months. The miners who work the remaining seams act as guides for the underground tours.

Blue John is a blue/purple and white/yellow banded variety of fluorite which locals will tell you is found nowhere else in the world other than this cave and the nearby Treak Cliff Cavern. However, blue fluorite occurs widely throughout Derbyshire and especially in the Ashover and Crich areas. It also occurs where other fluorspar deposits have been mined and so may be found in County Durham (especially Weardale), Cornwall and Wales as well as throughout the world. Indeed, similar banded blue and white/yellow flourite is now being imported into the UK from China and sold as coming from Castelton. Other deposits of banded blue fluorite occur in Nevada (in the USA) and Southern Iran. The Iranian blue-and-yellow-banded fluorspar has turned up in Roman grave-goods (e.g.as two cups near the Turkish/Syrian border) alongside the Persia to Rome trade route, and this has fed the myth that the Romans exported Blue John from Castleton. They did not. Neither, as often claimed, were two vases made from Castleton Blue John ever excavated at Pompeii.

In the late 18th century a small local industry was centred on turning vases of Blue John, which might be mounted with ormolu as chimneypiece garnitures. A small amount of the rock is still mined and can be bought as jewellery in local shops.

The name is popularly said to come from the French; bleu-jaune, meaning 'blue-yellow'. It is a fact that some Blue John was indeed sent to France for gilding by the French Ormolu workers of the Louis XVI period. However, they were emulating the pionerring ormolu ornaments of Matthew Boulton of Birmingham who around 1765 called the stone 'Blew John'. It became such a popular base for the ornaments that Boulton tried to lease the whole output of the Castleton mines.

The earliest dated decorative application of Blue John is its use in marble fireplace panels designed by Robert Adam and installed in Kedleston Hall near Derby in 1762.

It was featured on the 2005 TV programme Seven Natural Wonders as one of the wonders of the Midlands.