The cause of coloration in Derbyshire Blue John banded fluorite and other blue banded fluorites

banded fluorites from Ashover, Derbyshire, and Weardale, Co. Durham, has been investigated by a number of techniques, including mass spectrometry, optical spectroscopy, and paramagnetic resonance measurements on natural and irradiated samples. In all respects Blue John is indistinguish-able from the other blue banded fluorites. Although traces of hydrocarbons were found in all the natural fluorites including Blue John, they are shown not to be the cause of the colouration. The optical spectra nd bleaching behaviour are consistent with colouration by colloidal calcium rather than F-centres. The causes of colour banding are discussed. BLUE JOHN is the name given to the purplish-blue striated fluorite, CaF2, found in the Castleton area of Derbyshire and used for ornamental purposes. A detailed description of its occurrence and mineralogy has been given by Ford (I955, 1969). Since the earliest days of its use the cause of colour and banding in Blue John has been the cause of speculation, usually based on scant experimental evidence. Most writers have suggested, however, that the colouring agent in Blue John is unique and not related to that of the blue banded fluorites found elsewhere in Britain and abroad. Although much has been written on the coloration of fluorites in general