Castleton, a village in the Peak District just a few miles from our showroom in Sheffield, is the only place on Earth where the gemstone Blue John is found. This beautiful purple-blue and cream-yellow banded variety of the mineral fluorite is used both for jewellery and ornamentally and has been known since the time of the Romans, who took samples back to Rome for the Emperors to enjoy.
Fluorite is calcium fluoride and is also sometimes called fluorspar. The gem forms in a wide variety of colours, and often with bands of assorted colours within the same piece. Blue John is a multi-crystalline variety (formed of many crystal grains stacked together rather than a single crystal) of fluorite that has jagged bands alternating between dark purple-blue and light cream to yellow colours. It is spectacular when carved into vases and cups; such ornaments feature in the collections of many great houses and palaces. For jewellery it provides an attractive and unusual pattern in eye-catching pendants and earrings.
There are several explanations for the name ‘Blue John’. The most popular is that it comes from the French ‘Bleu Jaune’ indicating the distinctive blue and yellow colouration. An alternative idea is that local miners named the material, the name catching on from their slang: another mineral mined in the same area is sphalerite, which was known amongst miners as ‘Black Jack’ so it follows that a blue mineral found in the same area would be named ‘Blue John’. It is also possible that Cornish miners who journeyed to Castleton to work brought the name with them, as they already used it for a variety of minerals including fluorspar at home. Each story is plausible, and we shall perhaps never know which is the true origin of the name.
Even in the area around Castleton, Blue John is found in only two locations. One is Blue John Cavern and the other is Treak Cliff Cavern, one of several show caves in the area that are very popular tourist attractions. Here you can view the vein of Blue John in the rocky surface: specimens for use are extracted from areas of the cavern that are not on public display, with the vein in the show cave left for visitors to view. Material recovered from these locations is sold only to shops in Castleton (the material we sell comes from craftspeople with historical supplies). Only a limited amount is produced each year, due to both the scarcity of the mineral and agreements in place to protect supplies. A new vein discovered in 2015 was the first to be found in 150 years. Although not fully understood, the conditions believed to be required to produce the distinctive appearance of Blue John are complex and precise which explains why it is so rare.
Because Blue John is a very soft and brittle material it is usually bonded with resin before being fashioned into ornaments or pieces for jewellery. The resin fills in pores in the material and makes it more stable and durable. However, it should still be worn with care, and is more ideally suited to pendants and earrings than rings. With a hardness of only 4 on the Mohs’ Scale, fluorite is a soft mineral and Blue John’s brittleness only increases its susceptibility to damage. Worn with care though, Blue John can be displayed and enjoyed; it is a gem that is a real conversation starter.
You can browse a small selection of our Blue John jewellery on our website or visit us in our showroom to see more designs. Our experienced jewellers and gemmologists will be happy to talk to you about this fascinating English gem.