Blue John Cavern is one of the four show caves located close to Castleton in Derbyshire. It is named after the semi-precious mineral it contains, Blue John. This is unique to Castleton and is only found in one hill to the west of the village, nowhere else in the world.
The cavern was formed in the limestone around Castleton by water erosion. The Romans were the first people to discover Blue John, they exported it throughout their empire. Items made from Blue John were even discovered at Pompeii.
Blue John stone is a variety of Fluor-Spar, also known as Calcium Fluoride. It is found in veins about three inches thick, as well as nodular forms, lining the inner walls of cavities in the limestone.
The mineral was extensively mined during the Victorian era, because of this, it is now only mined in small quantities as the Blue John is running out. This is done by hand as blasting renders the Blue John worthless, the shock changing the colour making it an opaque off-white.
The cavern guides carry out the mining as well as cavern maintenance during the winter months.