Minerals / Blue John,(Fluorite). Treak Cliff cavern,

Blue john is a variety of fluorite, CaF₂ Calcium fluoride. The type locality for this mineral is Castleton in Derbyshire, and this unpolished specimen came from Treak Cliff cavern in Castleton. The unpolished surface reveals the crystal growth of the fluorite. Although Blue John is mostly  associated with Castleton, and is sometimes referred to as Derbyshire spar, similar deposits have been discovered in China.

 

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