Blue John Stone is a rare, semiprecious mineral found at only one location in the world – a hillside near Mam Tor, just outside Castleton in the Derbyshire Peak District National Park, England. 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.
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’.