Fabulous Fluorite: Derbyshire Blue John
Richard M Haw (UK)
Blue John is a unique variety of blue-purple banded fluorite. Hydrocarbons or oils have been deposited on some of the crystal surfaces while the mineral was forming. These oil layers are partly responsible for giving the stone an alternate blue and white banding, best seen when the stone is cut in section. It is not known to occur anywhere else in the world and is conﬁned to an area of about 1km³ of the Carboniferous “reef” limestones at Castleton in Derbyshire.
I have been involved with the public caverns here for a while and I am sure many of you have visited them. However, there are many people who have never even heard of Blue John, so the following article gives a general overview without intending to be too technical.
Castleton is a small village located in Derbyshire’s “Peak District” between the cities of Manchester and Shefﬁeld. The village is dominated by the ruins of Peveril Castle that was built by the Normans to oversee lead mining in the area. The scenery around Castleton forms a dramatic backdrop and the rolling limestone hills end abruptly atthe vertical face of Mam Tor. Beyond and to the north are the gritstone moors known as the “Dark Peak” that eventually lead up to the two-thousand-foot-high plateau of Kinder Scout.
Castleton and the surrounding area have been of interest to travellers, writers and scientists for hundreds of years. Lead mining goes back much further to Roman times and, at the foot of Mam Tor, is the Odin Mine. It is believed to be the oldest lead mine in Britain having Saxon origins. For centuries, Derbyshire’s lead deposits were of worldwide importance but lead mining in the region had ceased by the early 1900s due to market forces. Mining for other minerals, mainly ﬂuorite, baryte and calcite still continues today in the hills above the village and there has also been recent talk of re-opening or re-working some of the old lead mines.
Castleton is also famous for its natural caverns, four of which are open to the public as show caves. It is believed that it was within two of these natural show caves that the rare, blue-purple banded fluorite, known as “Blue John”, was ﬁrst discovered.
A brief history of Blue John
What we do know is that Blue John has been mined for its ornamental use since about 1700 and, by turning it on a lathe, it was made into large vases, candelabra, urns and bowls. However, there are stories that the Romans mined the mineral, although there is no hard evidence to support this.
The stone was largely made popular by the architect Robert Adam, who used Blue John in thin slices as an inlay in his ornate marble ﬁreplace surrounds, and by his colleague Matthew Boulton, who made fine ornaments and clocks incorporating Blue John.
By the 1780s, the craft in Blue John ornaments was the very height of fashion, especially among aristocracy and the wealthy. Many articles were commissioned for the large stately homes being built at that time throughout the country. Some of these survive today and can be seen in numerous houses like Windsor Castle, Chatsworth House and Kedleston Hall, to name but a few. The stone has also travelled further a ﬁeld – the Vatican in Italy has a fine Blue John window and ornaments of Blue John adorn the Oval office in the White House. The Natural History Museum in London is now home to the largest Blue John vase ever made, standing over two feet tall, as well as some other very fine specimens.
The Blue John trade represented the pinnacle of British manufacturing and craftsmanship in the late 1700s. It was a showcase to the World that England was capable of producing fine ornamental works of art that matched, if not exceeded, those coming from the Continent, especially from Italy, France and Greece.
However, Blue John’s fame and fashion was short lived. Soon, the pieces of stone required to make large articles became mined-out. A steady stream of smaller articles like pillboxes, candlestick holders, paper weights and bowls continued to be produced until late Victorian times but, even then, large pieces of good quality stone were becoming scarce. The Victorians introduced the manufacture of Blue John jewellery and, since this did not require large pieces of stone, has been sustained to the presentday.
Today, there is still a fair demand for Blue John and, occasionally, a piece of stone large enough to make a bowl or vase is found. However, these are rare and the amount of stone mined each year is probably less than 500kg.
Derbyshire is divided into two general, geological regions. To the south, the “White Peak” comprises mostly Carboniferous limestones, and to the north, the “Dark Peak” is mainly shales and coarse sandstones known as “gritstone”. It is generally thought that, during Carboniferous times (330Ma), the region was a shallow, tropical, marine environment roughly on or near the equator. It had some reefs and some deeper waters, and the occasional volcano or volcanic vents spewed lava onto the sea ﬂoor depositing thin bands of basalt. This environment lasted for a period of about 40 million years and formed limestone beds some 3km thick.
The Castleton area was then subject to uplifting that raised the limestones above sea level and subjected them to erosion. Not only did surface eroding take place, a complex series of small caverns, known as “pipes”, were created by the natural process of “karst” cavern formation.
The region was then re-submerged beneath the sea and was buried by shales and sandstones. These shales and sandstones formed a cap on the limestones trapping ﬂuids within them.
Then, about 270Ma, as a result of major movement within the Earth’s crust, the whole area started to be pushed back up again, forming what we know today as the “Pennine Anticline”.
This uplifting caused the rocks to crack and opened gaps along a roughly east-west directionthat were ready to be ﬁlled with minerals. Low temperature (130 to 200°C) hydrothermal fluids trapped deep underground ﬂowed into the joints, cracks and pipes in the limestone, depositing fluorite, calcite, baryte and galena. Surface erosion and the formation of a second, large natural cavern system at the end of the last ice age series brings us to the present day.
The caverns and mine today
The Blue John Caverns and Mine have been a visitor attraction since the 1770s when the first “tours” were taken. Descending nearly three hundred feet using miners’ ladders with only candles for lighting, the journey was dangerous and took several hours. The Victorians constructed the present tour route by blasting an improved entrance. They laid proper paths and replaced the miners’ ladders with stone steps. Improved lighting was introduced with the use of flares and gas lamps and, other than electric lighting that was installed in the 1960s, very few changes have been made to the show caverns since the 1840s.
Mining continues today during the winter months and generally involves re-working the existing deposits of Blue John stone. Each particular deposit or vein of Blue John has its own unique pattern and characteristics with 14 of the larger vein shaving their own names: Old Dining Room Vein, Millers Vein, Five Vein and Bull Beef, to name but a few. These names were probably given by the old miners for reference but were later adopted as a sales gimmick for finished items. Some craftsmen requested particular veins for making certain objects.
What was dificult and time-consuming to remove with a hand drill two hundred years ago can now be removed much more efficiently and quickly using modern power tools. However, extraction of the Blue John from the limestone is still difficult and, even today, is a slow and careful process.
The fluorite is brittle and can easily be damaged. Techniques were developed in the 1700s to release the Blue John by chipping away the excess limestone rock with a hand pick, and then drilling rows of holes into the remaining surrounding limestone. These holes were then filled with either wood or quicklime that was then wetted and left to expand. This method caused splitting of the limestone rock without impact damage to the Blue John. Today’s method is very similar with the exception that we now use a “Kango” drill instead of a hand drill and we can use expanding “rawl bolts” to split the rock.
Today, before mining into the walls, the whole area is “sounded out” and checked for loose rock, structural defects and the ﬂoors are checked for old collapsed mine workings. Some of these checks then take place every 15 minutes or so once the drilling begins and it may take several days of careful drilling to remove a large piece.
Natural erosion of the limestone during the formation of the caverns is also useful from a miner’s perspective. Blue John was detached from the vein or pipe walls by the limestone dissolving away behind it. These loose lumps were then washed into the sediments in the cavern floor. Many winter mining seasons have been spent washing through the thick clay in the cave floors for these detached pieces, known as “Riders” and as much as 60% of the Blue John mined today is obtained by this method.
The early spoil heaps can also be host to some small but ﬁne pieces of Blue John. Generally, only small chunks are found, as only large pieces were required in the 1700s. However, these tend to be good quality and anything larger than about walnut size is recovered.
Once mined, all the stone must be carried by hand to the surface. Unfortunately, there is no easy way of doing this today. It is generally carried in a backpack, although very large pieces tend to be lashed to a pole or bar so two or three of us can carry it up to the surface together.
The spoil tips also contain some excellent mineral specimens. Colourless, yellow, red, orange and even black fluorite crystals are found. There are ﬁne calcite and baryte specimens by the ton and, sometimes, small pieces of galena are seen. Even stalactites, removed by the old miners or destroyed by natural collapse turn up, as well as old tools, bottles, and bits of old boots, and sometimes even bone!
In some of the old mine workings, the walls are adorned with inscriptions of the miners. Initials and dates going back to the eighteenth century can be seen, looking as fresh as the day they were carved.
In several parts of the old mines, candle holders and chisels sit on ledges, undisturbed for centuries. In one part, the old miners’ wooden ladders can be seen in place down a shaft, descending into the darkness, these lead into workings not explored for over a hundred years. Who knows what is down there? Maybe, in time, we will get chance to explore. We may even find more caverns that undoubtedly exist. The caving groups that sometimes visit us are gaining access into new areas every year. Universities and geologists are studying the area all the time and continue to make new discoveries. Who knows what the future holds for Blue John and its caves and mines, but it all adds to the charm and mystery of this amazing and historic mine.
For anyone wishing to know more of this fascinating subject, I recommend a visit to the caverns. There are also a few good books on the subject, but I the one I recommend is the excellent Derbyshire Blue John by Trevor D Ford that covers, in detail, all aspects of the topics merely touched upon in this article. Recent TV programmes like the Antique Roadshow, Flog-it! and Bargain Hunt have brought Blue John back into the limelight and once again the stone is becoming very popular and collectable. Both mineral and ornament, newpieces or antiques, all represent an excellent investment and its value can only increase as the stone becomesscarcer.
Derbyshire Blue John (2nd edition) by Trevor D Ford, The East Midlands Geological Society, Nottingham (2019), 80 pages (softback), ISBN: 97800951971796.