Monday, November 03, 2008

Blue John

Chatsworth, Derbyshire UK


Above: A Blue John Vase in the state bedroom at Chatsworth House

Blue John Stone is a rare, semiprecious mineral found at only one location in the world - a hillside near Mam Tor, just outside Castleton. The name Blue John derives from the French Bleu Jaune meaning Blue Yellow, it is a form of fluorite and was discovered as miners were exploring the cave systems of Castleton for lead.

Blue John fluorite has been worked into ornaments since 1750. Within ten years there were sixteen mines working the area to supply thirty-odd local firms who fashioned it into inlays for fireplace and other ornaments for the stately homes of England. Old craftsman dried the Blue John stone and noticed that it changed colour to pink and red. It is not clear if this was discovered by accident as a result of a mistake in drying the stone. At 230°C the Blue John stone changes colour but at these temperatures, gases are produced which can sometimes explode and destroy the piece.

Nowadays, the caves of Castleton are magnificent show caves and are some of the most popular tourist attractions in Derbyshire. Of the four show caves only Treak Cliff and, to a lesser extent, Blue John have veins of Blue John Stone. Treak Cliff Cavern still mines about 500 kilograms of Blue John Stone each year. The veins of Blue John Stone are easy to see and many of the formations are well lit. Blue John Stone is a semiprecious stone and gives Castleton its nickname of 'Gem of the Peaks'.

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