Blue John Stone
Blue John Stone is a rare, semiprecious mineral found at only one location in the world - the Treak Cliff hillside just outside Castleton. Discovered by lead miners in search of their much-prized ore, Blue John samples were sent to France where the top jewellers worked. They called it "Bleu Jaune" meaning Blue Yellow - the English changed this to Blue John!
Chemically, Blue John is calcium fluoride (CaF2) which has been coloured by films of oil deposited on the crystals millions of years ago. Although it is known as fluorite the old miners, referred to it as fluor spar (sometimes spelt fluorspar). 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.
Working Blue John is very tricky as it is very friable. Jewellers today boil the stone repeatedly in pine resin, which holds it together whilst it is worked. Banded Blue John fluorite has been worked into ornaments since 1750 and there are about 14 categories of banding patterns. The stone is highly prized for ornaments and jewellery and it can be found in many great collections housed in places such as Chatsworth, Windsor Castle, the White House and the Vatican.