Blue John stone is the name given to banded fluorite found in the Castleton area of Derbyshire in England (Ollernshaw, 1964). It has been prized for centuries. Chemically, it is a calcium fluoride (CaF2) and occurs in distinct bands of different colours: blue, white, purple and yellow. The colour banding is thought to be from periodic changes in the composition of the mineralising solution and the physical conditions during its formation (Mackenzie and Green, 1971). The name of this distinctive material is thought to have come from the French “bleu et jaune”, referring to its blue and yellow colours.