Blue John (also known as Derbyshire Spar) is a rare, crystalline, semi-precious mineral, a form of fluorite unique to a single mine at Castleton, Derbyshire. This particular form of fluorite, much admired for its characteristic bands of a purple-blue or yellowish colour, was mined intensively in the 18th and 19th centuries for its ornamental value. The name may have derived from the French bleu-jaune, meaning 'blue-yellow'. By repute, Blue John was exported to France where it was used by ormolu workers during the reign of Louis XVI (1774–1791). 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.