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Its existence at Treak Cliff in Castleton, Derbyshire, was rumoured nearly 70 years ago but it has now been revealed by John Turner, 21.

In 1945 a miner called John Royse claimed to have located a large vein of the stone and promised to tell Peter Harrison, whose family had taken over the running of the Treak Cliff Cavern, about the exact spot, but died before he could do this. Mr Turner is the grandson of Mr Harrison, now 87.

The stone - a unique banded form of fluorspar - can only be found in Derbyshire.