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. Fig. 1. Old picture taken sometime in the 1870s, showing miners digging in the Old Dining Room, now part of the show caves. 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. The area 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 … Read More

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