Castletown, Derbyshire - Crystal Waterfall, Blue John Cavern - real photo card

Postcard
• Picture / Image:  Crystal Waterfall, Blue John Cavern, Castleton, Derbyshire - real photo type
• Publisher:  none given
• Postally used:  no - brief notation
• Stamp:  n/a
• Postmark(s): n/a
• Sent to:  n/a
• Notes / condition:  slight wear to edges and corners and a drink ring on reverse

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Text from the free encyclopedia WIKIPEDIA may appear below to give a little background information (internal links may not  work) :

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The Blue John Cavern is one of the four show caves in Castleton, Derbyshire, England. The cavern takes its name from the semi-precious mineral Blue John, which is still mined in small amounts outside the tourist season and made locally into jewellery. The deposit itself is about 250 million years old.

The miners who work the remaining seams are also the guides for underground public tours. The eight working seams are known as Twelve Vein, Old Dining Room, Bull Beef, New Dining Room, Five Vein, Organ Room, New Cavern and Landscape.

The cavern was featured on the 2005 TV programme Seven Natural Wonders as one of the wonders of the Midlands. In the 2010 series of How it's Made, the cave and jewellery production of Blue John was featured.

In the UK Blue John, or ""Derbyshire Spar"", is found only in Blue John Cavern and the nearby Treak Cliff Cavern. It is a type of banded fluorite. The commonest explanation for the name is that it derives from the French bleu-jaune, meaning 'blue-yellow', but other derivations have been suggested

Derbyshire Blue John (also known as Derbyshire Spar, or simply Blue John) is a semi-precious mineral, a 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.

The most common explanation for the name is that it derives from the French bleu-jaune, meaning 'blue-yellow'. The story goes that Blue John was exported to France where is was used by ormolu workers during the reign of Louis XVI (1774–91). However, there is no archival record of any Blue John being exported to France,^[1]:7 and the early ormolu ornaments which use Blue John were being manufactured by Matthew Boulton of Birmingham in the 1760s.

An alternative origin of the name derives from an old miners' name for the zinc ore sphalerite which they called ""Black Jack"". Thus the unique blue stone mined in these caverns could easily have become known as ""Blue John"".^[1]:7

Another derivation comes from the Cornish miners who began working the Derbyshire lead mines in the 1740s. The name Blue John is used for several rocks in Cornwall, including fluorspars, and derives from the Cornish language word bleujenn, in Old Cornish blodon, a flower, bloom or blossom.^[2]

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 town of Castleton.^[1]: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.^[1]:36

The mineral veins of the Peak District were formed during the late Carboniferous and early Permian times, when the limestones were at a depth of 3 kilometres (1.9 mi).^[1]:13 The minerals were deposited in veins by layers of crystals precipitating from hot fluids coating the walls of fractures, caves, and other cavities.^[1]:17 Petrological analysis has shown that the Blue John, like fluorite elsewhere in the Peak District, crystallised from a highly saline fluid at temperatures of 90–120°C or perhaps a little higher.^[1]:25

Despite much investigation, the origin of the blue colour of Blue John remains uncertain. Microscopic analysis has failed to find any impurities such as Potassium permanganate or hydrocarbons which could produce a purple-blue colour. It is now thought that the colour may be a physical phenomenon due to crystal lattice dislocation. If the regular arrangement of atoms in the fluorite molecules are disturbed or dislocated, then this may yield the blue colour in Blue John.^[1]:27 The cause of these dislocations is unknown, but one possibility is that it is due to colloidal calcium, i.e. excess of calcium atoms needed to form fluorite.^[1]:28 Blue John can be decolourised by heating it in an oven for a few hours, a phenomenon apparently caused by the heat realigning the lattice dislocations removing the colour. Irradiating the discoloured Blue John in an atomic reactor can bring the colour back.^[1]:29

type=real photographic (rp)

theme=topographical: british

sub-theme=england

county/ country=derbyshire

number of items=single

period=1945 - present

postage condition=unposted