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.
Etymology
The most common explanation for the name is that it derives from the French , meaning 'blue-yellow'. The story goes that Blue John was exported to France where it 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, 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".
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 , in Old Cornish , a flower, bloom or blossom.
Geology
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. 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.
Mineralogy
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 . The minerals were deposited in veins by layers of crystals precipitating from hot fluids coating the walls of fractures, caves, and other cavities. 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.
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. 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. 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 a nuclear reactor can bring the colour back.
Similar rocks elsewhere
Blue, unbanded fluorite occurs in many localities around the world. Within the UK, blue fluorspars are also found in County Durham, especially Weardale. Elsewhere, blue fluorspar is known in the Ardennes region of Belgium; the Cave-in-Rock area of Illinois in the United States; and at various localities in Mexico and China.
Blue-banded fluorite is very rare. An ornamental mineral known to the Romans as murrhine may have been a fluorspar similar to Blue John. Recently discovered deposits in China have produced fluorite with colouring and banding similar to the classic Blue John stone. One source is the Deqing Fluorite Mine, in Deqing County, Zhejiang Province, the only source (other than in Derbyshire) that has an ornamental craft based upon it. However, although this fluorite is similar to Blue John its banding is straighter and it contains colours not associated with Blue John, suggesting they are compounds and not inclusions as in Blue John.