One of the most famous of the older-known localities of fluorite is Castleton in Derbyshire, England, where, under the name of Derbyshire Blue John, beautiful purple-blue fluorite was extracted from the Blue John Cavern. It was used for ornamental purposes, especially in the 19th century. The name derives from French "bleu et jaune" (blue and yellow) characterising its colour. It is now scarce, and only a few hundred kilograms are mined each year for ornamental and lapidary use. Recent deposits in China have produced fluorite with similar colouring and banding to the classic Blue John stone.
The entry for "Blue John Cavern" has this to say about the name:
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.
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