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Image courtesy of Roy Starkey
Fluorite, Blue John
Donor: Unknown
Dimensions: 125x105x4
Place Object Found: Castleton
Department: Earth Sciences
Accession Number: U1310
Blue John is a type of banded, purple Fluorite, found only in a small part of the Peak District in Derbyshire. How this type of fluorite formed its unusual colouration is still debated. Several possibilities, such as impurities and natural faults in the crystal structure, have been suggested and the truth is most likely a complex combination of all of them. The crystals began to form deep under the limestone rocks of the Peak district, around 300 million year ago, as chemicals from highly saline liquids precipitated onto the walls of cavities and caverns.
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