... of interest well worth seeing at Castleton besides the great cavern; there was the famous Blue John Mine, that took its name from the peculiar blue stone found therein, a kind of fibrous fluor-spar usually blue to purple, though with occasional black and yellow veins, of which ornaments were made and sold to visitors, and from which the large blue stone was obtained that formed the magnificent vase in Chatsworth House, the residence of the Duke of Devonshire, and in other noble mansions ... — From John O'Groats to Land's End • Robert Naylor and John Naylor
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