Alchemy Best of British
Derbyshire Blue John jewellery. Derbyshire Blue John is the name given to a distinctive colour banded type of polycrystalline fluorite found in one side of one hill in the Peak District near Castleton.
The mineral veins of the Peak District were formed during the late Carboniferous/Early Permian times when the host limestones were at a depth of 3 kilometres. The minerals were deposited in veins by layers of crystals precipitating from hot fluids coating the walls of fractures, caves, and other cavities. Petrological analysis has shown that the Blue John, like fluorite elsewhere in the Peak District, crystallised from a highly fluid at temperatures of 90–120 °C. The origin of the blue colour of Blue John remains uncertain. Colour in gemstones is often caused by impurities such as potassium permanganate or fluid hydrocarbons inclusions however no such cause has been identified in this case. It is now thought that the colour may be a physical phenomenon due to crystal lattice dislocation. Light reacting to these dislocations can cause a blue colour. The cause of these dislocations is unknown, but one possibility is that it is due to colloidal calcium, i.e. excess of calcium atoms needed to form fluorite. Blue John can be decolourised by heating it in an oven for a few hours, a phenomenon apparently caused by the heat realigning the lattice dislocations removing the colour. Irradiation causes the colour to return.
Derbyshire Blue John is still mined at both Treak Cliff Cavern and Blue John Cavern on the outskirts of the town of Castleton in the Derbyshire Peak District. Both mines operate underground visits during which visitors can learn about the history of mining as well as observe some of the underground geological wonders on show.
Here at The Ebor Jetworks we like to combine this beautiful Derbyshire Blue John with our own Whitby Jet to give our own unique look to both these traditional British gem materials. These can be found in our Alchemy collections as part of our Best of British range.