Fluorite variety Blue John, Trek retailer Cliff, Castleton, Derbyshire

Blue John is a semi-precious mineral, a rare form of fluorite with bands of a purple-blue or yellowish colour. In the UK it is found only in the Caverns around Castleton in Derbyshire. During the 19th century, it was mined for its ornamental value, and mining continues on a small scale. Despite much investigation, the origin retailer of the blue colour remains uncertain.Microscopic analysis has failed to find any impurities which could produce a purple-blue colour. It is now thought that the colour may be a physical phenomenon due to crystal lattice dislocation.  The cause of these dislocations is unknown, but one possibility is that it is due to colloidal calcium, i.e. excess of calcium atoms needed to form fluorite.

This piece is from the Trek Cliff Cavernand weighs in at 145g and is 7x6x5 cm with crystals to 4mm.