Blue John

A rare semi-precious fluorite mineral

"Roman legend believes that the eminent properties of the Blue John keep them away from drunkenness when they drink from the cups or bowls made of it."
— Gemexi

Blue John is a rare form of an otherwise common mineral, fluorite. It's rarity is down to the colour banding, which as the name suggests, tends to the purple, but ranges into both a dark blue and yellow. In fact this colouration could be a source of its name, from the French bleu-jaune, which means "blue-yellow"; other etymologies are available, including a Cornish word bleujenn, (blodon in Old Cornish) meaning flower.

I first stumbled upon this mineral in a tourist gift shop in Bakewell, during a week of wandering through Derbyshire. It really is a beautiful thing, the dark crystal waves blending into white, with occasional streaks of lemon-yellow. Little wonder it was, and is, much in demand for all sorts of work from making jewellery to ornamental dishes.

Blue John (also known as Derbyshire Spar) is a semi-precious mineral, a rare form of fluorite with bands of a purple-blue or yellowish colour. In the UK it is found only at Blue John Cavern and Treak Cliff Cavern at Castleton in Derbyshire. During the 19th century, it was mined for its ornamental value, and mining continues on a small scale. It is not a hard mineral; Segnbora-T points out that flourite ranks quite low on the Mohs hardness scale. In the United Kingdom, the blue, banded fluorite known as Blue John is found only under the triangular hill known as Treak Cliff, just outside the Derbyshire town of Castleton. 20  Today the veins are mined only in Blue John Cavern and the nearby Treak Cliff Cavern, although the abandoned Old Tor Mine on the north side of Winnats Pass was also, at one time, a source for the mineral.

Occasionally a fortunate hiker may find a pebble or fragment on their walk. I once collected a nubbin whilst walking on Old Mam Tor Road, but it has long migrated to The Land of Lost Things, almost certainly during a move.

Images of Blue John

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