Derbyshire Blue John & Whitby Jet

Blue John is a rare form of fluorite occurring only in a series of veins in the limestones of Treak Cliff, a hill near Castleton in Derbyshire. Pure fluorite is colourless but when contaminated by other minerals can be green, yellow, pink or purple. Blue John however is banded dark blue, purple, white and yellow and its origin has long been a puzzle to mineralogists . Hydrocarbon minerals derived from organic remains within the limestones occur in the Castleton area and although conflicting opinions exist, it is generally considered that the banded character of Blue John is due to the imprisonment of hydrocarbon compounds within the fluorite at the time of it’s formation. Although little evidence exists to indicate the mining of Blue John prior to C17th, vases found in Pompeii made from a material similar to Blue John may indicate that the Romans knew of it’s existence. By the C18th it was prized as an ornamental and decorative stone. Robert Adams famously used it in his fireplaces.

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