BLUE JOHN FLUORSPAR

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Abstract
Summary: The paper presents an account of the mineralogy, geological occurrence and history of the blue and white banded fluorspar found near Castleton in Derbyshire. Blue John is shown to be a fluorspar, coloured by bituminous inclusions, deposited as cavity-linings and replacements in Carboniferous Limestone of reef and massif facies. Alternative hypotheses of the cause of the colouring in Blue John are discussed. Lying more or less parallel to the bedding of the limestones, the deposits are in pipe-veins rising up the flanks of a pseudo-anticlinal structure formerly covered by impervious Namurian shales, which now outcrop only round the flanks of Treak Cliff. The relationships between Blue John and the associated minerals (calcite, barytes, galena, pyrites and quartz) are described, and an account is given of the history of the use of Blue John and the methods of manufacturing ornaments. Reasons are given for the conclusion that the story of the use of Blue John by the Romans is without foundation.

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