Blue John: Remarkable fluorite from a limestone cavern
Steven Wade Veatch (USA)
Blue John stone is the name given to banded fluorite found in the Castleton area of Derbyshire in England (Ollernshaw, 1964). It has been prized for centuries. Chemically, it is a calcium fluoride (CaF2) and occurs in distinct bands of different colours: blue, white, purple and yellow. The colour banding is thought to be from periodic changes in the composition of the mineralising solution and the physical conditions during its formation (Mackenzie and Green, 1971). The name of this distinctive material is thought to have come from the French “bleu et jaune”, referring to its blue and yellow colours.
Blue John is mined from only two places – Treak Cliff Cavern and Blue John Cavern in Castleton. It occurs either in veins up to 7.5cm thick or as nodules in a limestone unit found inside natural caverns beneath a hill west of Castleton. The caverns are now tourist attractions, where visitors can go on underground tours (British Council, 2008).
Castleton is an excellent example of a quintessential English town. A beautiful stream quietly flows through this picturesque community of quaint tea shops, inviting pubs, charming cottages and old stone houses. Peveril Castle is a short walk up the hillside.
Blue John was first discovered about 2,000 years ago when the Romans mined lead and other ores in the caverns of Castleton. The Romans valued its beauty and mined it for use in ornamentation.
Two vases made of the stone were unearthed during archaeological excavations at Pompeii. Artefacts made of Blue John have been found at other ancient Roman sites and there are several references to the stone in Roman literature. It was also in demand in the 18th and 19th century for ornamental vases and columns used in some of the finest houses in Britain.
|Did you Know?|
|Under ultra-violet light (UV), Fluorite is well known to fluoresce. The visible light emitted is often blue, but can also be red, purple, yellow, white and green. However, this is not strictly true for all types of Fluorite. Much depends on where the miner al comes from. The best fluorite samples in the UK for viewing under UV light are found in Northumberland, County Durham, and Eastern Cumbria. Specimens from Derbyshire including Blue John, and indeed Yorkshire and Cornwall actually rarely fluoresce, and if they do, generally are quite poor.|
When mineralogists first discovered the fluorescent properties of fluorite, they believed that they could measure the light emitted, and determine where a specimen comes from. It was believes that UV light would produce a fingerprint of the mineral. However, although specimens from similar locations do share similar UV reactions, they quickly discovered that actually, each specimen even from the same locality can vary, particularity in the brightness of fluoresce.
Today, the larger Blue John veins are largely mined out and the material is now scarce – only a few hundred pounds in weight are mined each year for specimens, ornamental pieces and for jewellery sold in shops in Castleton.
British Council, 2008. “Going down the mine.” British Council Web site: Retrieved March 21, 2009, from http://www.britishcouncil.org/learnenglish-holiday-countryside-peaceful.htm.
Mackenzie, KJD and Green, JM 1971. The cause of Color in Derbyshire Blue John banded fluorite and other blue banded fluorites. Mineralogical Magazine, December 1971, vol. 38, pp. 459-470.
Ollernshaw, AE 1964. The history of Blue John stone; methods of mining and working, ancient and modern. Published Castleton via Sheffield, 23 p.