|Blue John Cavern|
Entrance to the Blue John cavern
The Blue John Cavern is one of the four show caves in Castleton, Derbyshire, England. The cavern takes its name from the semi-precious mineral Blue John, which is still mined in small amounts outside the tourist season and made locally into jewellery. The deposit itself is about 250 million years old.
The miners who work the remaining seams are also the guides for underground public tours. The eight working seams are known as Twelve Vein, Old Dining Room, Bull Beef, New Dining Room, Five Vein, Organ Room, New Cavern and Landscape.
The cavern was featured on the 2005 TV programme Seven Natural Wonders as one of the wonders of the Midlands. In the 2010 series of How it's Made, the cave and jewellery production of Blue John was featured.
Blue John 
In the UK Blue John, or "Derbyshire Spar", is found only in Blue John Cavern and the nearby Treak Cliff Cavern. It is a type of banded fluorite. The commonest explanation for the name is that it derives from the French bleu-jaune, meaning 'blue-yellow', but other derivations have been suggested.