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Tucked away at the head of the Hope Valley - one of the glaciated valleys in the Peak District National Park - lies the little community of Castleton, Derbyshire. Although the beauties of the first National Park ever established in Great Britain are all around visitors to this lovely spot, it is two things that dominate Castleton. As you can imagine, given the village's name, there are the ruins of a splendid Norman castle perched high above the settlement on a ridge - which is, actually, a fossilized barrier reef - overlooking Cave Dale.

The name of Cave Dale gives you a clue to the other major attraction in Castleton. The hills are riddled with caves and caverns - including the deepest shaft in Great Britain, 'Titan', a 464 feet vertical drop. Under this one little mountain is found a most wonderful semi-precious mineral, a former of banded flourspar, called by the local name, 'Blue John'. This halite mineral has the chemical formula, CaF2, or calcium flouride, and is found as crystalline deposits in the limestone which forms these hills. The mineral was formed at the end of the Carboniferous period by the injection of a hot, super-saturated solution of salts into cracks or fissures in the limestone.

Blue John is made up of alternating bands of purple, yellow, deep indigo blue, and sometimes white or pink/red. At first, these colors were supposed to be due to trace elements, but it is now thought that they are caused by a crystallographic defect or dislocation, which gives a physical effect. This hypothesis is supported by the fact that these colors can be 'destroyed' by heat, and 'recovered' by irradiating the crystal in an atomic reactor.

At first, it was supposed that the name 'Blue John', came from the French, 'bleu-jaune', or 'blue yellow', but there is an alternate theory, that Cornish miners, who came to work the mines in the 1740s, might have called the fluorspar, 'bleujenn', in the Cornish language, which referred to the flower-like appearance of some Cornish flourspars. Personally, I prefer the French explanation!

There is a great deal of debate about whether or not the Romans, who occupied this area, built forts and formed a mining consortium, 'Lutudarum', to handle the production of lead, (galena, an ore of lead, is found in quantity at the southern edge of the Carboniferous Limestone) would have sent examples of the beautiful crystal to Rome or other cities of the Empire. Some authorities say no, local Derbyshire opinion says yes! Lead and lead/silver ingots were exported to other parts of the Empire, particularly during the reign of the Emperor Hadrian, 117CE to 138CE, possibly through the Roman port of Petuaria (now Brough-on-Humber) to the east; lead ingots, or 'pigs', with the Derbyshire mark, have been found as far away as Normandy. This location of Petuaria was important, as no less than eight rivers - the Ouse, Hull, Derwent, Wharfe, Trent, Ancholme, and Aire - all giving access far inland, can be reached from this point.

Blue John became wildly popular in England, during the 18th century, with the eminent architect, Robert Adam (1728 - 1792), producing panels and whole fireplaces for the stately home, Kedleston Hall, on the outskirts of Derby. Chatsworth House, in the Peak District - the seat of the Dukes of Devonshire - has some fine pieces, and HRH Queen Elizabeth has a room in Buckingham Palace which is decorated with many examples.

Unfortunately, the Victorians used explosives to extract the mineral, thereby spoiling a lot of the best deposits. Indeed, the Blue John that is used for jewellery production, today, comes from sorting through the gigantic underground spoil heaps left by the Victorian miners. Blue John is extracted from Treakcliff Cavern and Blue John Cavern; the Old Tor Mine located at Winnats Pass, just outside the village proper, has now been abandoned.

The building you can see here - built in local gritstone - is home to A.S.D. Jewellery Ltd. This family-owned business was founded in 1977, and is one of four outlets in the village which sell Blue John. A.S.D. prides itself on using only local craftsmen, with all work being done on the premises, including repairs. They really do produce fine quality pieces, with modern, traditional and Celtic designs. Your Blue John piece can, literally, be made to order, and set in 9 carat or 18 carat yellow, white or rose gold, or in solid silver. If you want my opinion, (and it is purely personal), then a silver setting is much more appropriate for the colors of the stone. I have to admit, that since Castleton is approximately half-way between Manchester International Airport (ATA: MAN, ICAO: EGCC) and some of my family in Sheffield, this is a natural place for me to stop off and purchase gifts for staff members in the hospital I work in! In my family, and many others in Derbyshire, it is customary to give the bride, maid of honor and all the bridesmaids, gifts of Blue John jewellery on the day of the wedding. A lovely custom.

It would appear that colored, banded fluorspar is now being extracted from the Deqinq Flourite Mine in Zhejiang Province, China. If we are not careful, the market could be flooded with cheap Chinese Blue John!

Blue John, a lovely stone, and one to treasure.

Originally posted to shortfinals on Sun Nov 03, 2013 at 07:37 PM PST.

Also republished by History for Kossacks and Derbyshire and The Peak District.

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