Blue John (mineral)

The most common explanation for the name is that it derives from the French , meaning 'blue-yellow'. It is said that Blue John was exported to France where it was used by ormolu workers during the reign of Louis XVI (1774–92). 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". ==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 village of Castleton. 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. One source is the Deqing Fluorite Mine, in Deqing County, Zhejiang Province, but these vases are in fact made from banded amethystine quartz. ==Discovery==

It is sometimes said that Blue John of Derbyshire was discovered by the Romans. However, the earliest source of this story is William Adam's 1843 book Gem of the Peak. It seems likely that Adam was attempting to add some mystique to the Derbyshire Blue John. Although the Romans did mine lead in Derbyshire, there is no evidence that they encountered the Blue John veins. The earliest reference to the mineral "Blue John" occurs in a letter dated 1766 noting a lease from Lady Mazarine, which states that she "let ye Blue John, Castleton". The next reference occurs in a 1768 letter by the industrialist Matthew Boulton who attempted to purchase or lease the mines so that he could mine the Blue John to create decorative vases. It is known that by this date the mining of Blue John had been going on for several years. The earliest dated decorative applications of Blue John in Britain are those in use as fireplace panels. The bridal suite of the Friary Hotel in Derby has a Blue John plaque dated to around 1760. About the same time, fireplaces with Blue John panels were designed by neoclassical architect and interior designer Robert Adam, and put in place at Kedleston Hall near Derby. ==Later output==

By the 19th century Blue John was being fashioned into a wide variety of ornamental items ranging from knife-handles to chalices. and in 2015 a new vein, the first for 150 years, was discovered close to the tourist route in the same cavern. Elsewhere, similar blue and white-yellow banded fluorite ornaments are now imported into the UK and other countries from China. == Production ==

Before they can be worked, the stones (having been air-dried for at least a year) are heated in an oven, then placed in a bowl of hot epoxy resin (previously, pine resin was used), and then further heated in a vacuum oven. This drives out air from minute pores in the stone, and replaces it with the resin, which binds the otherwise friable crystal structure, allowing it to be cut and polished. After resining, the stones are cut on a saw. They may be made into rough cuboids or cylinders ("rough-outs"), for turning as bowls and vases, or flat slices, for making jewellery. Rough-outs are glued to a metal chuck and turned on a lathe, sometimes using pieces of broken grinding wheels. The chuck is removed by heating the glue, or—if the operator is inclined—a sharp tap on the chuck with a spanner. A further resining stage may take place, before the piece is returned to the lathe and polished with wet abrasive paper. A final high polish is added using putty powder (finely crushed tin dioxide) applied with a moist piece of felt. File:Raw Blue John from Treak Cliff Cavern.JPG|Blue John in its natural state File:Heating Blue John.JPG|Heating stones in the oven File:Blue John production - 1 resin - Treak Cliff Cavern - Andy Mabbett - 15.JPG|Hot stones in a vat of hot resin File:Blue John production - 1 resin - Treak Cliff Cavern - Andy Mabbett - 03.JPG|Unresined (left) and resined (right) Blue John File:Blue John production - 2 sawing - Treak Cliff Cavern - Andy Mabbett - 02.JPG|Sawing a block File:Blue John production - 3 turning - Treak Cliff Cavern - Andy Mabbett - 03.JPG|Mounted block File:Blue John production - 3 turning - Treak Cliff Cavern - Andy Mabbett - 04.JPG|Part-worked piece on the lathe File:Blue John production - 3 turning - Treak Cliff Cavern - Andy Mabbett - 12.JPG|Working a piece on a lathe File:Blue John production - 3 turning - Treak Cliff Cavern - Andy Mabbett - 29.JPG|Polishing File:Treak Cliff Cavern - Blue John ornament - Andy Mabbett - 12.JPG|A finished ornament For making jewellery, thin slices are marked out and cut into shapes such as circles or ovals, then finished on a grinding wheel. The rear faces of single-sided pieces are painted white before they are mounted. File:Blue John production - 4 jewellery - Treak Cliff Cavern - Andy Mabbett - 01.JPG|Marking a slice of Blue John File:Blue John production - 4 jewellery - Treak Cliff Cavern - Andy Mabbett - 05.JPG|Shaping a jewel File:Blue John production - 4 jewellery - Treak Cliff Cavern - Andy Mabbett - 07.JPG|Painting the rear face File:Treak Cliff Cavern - Blue John jewellery - Andy Mabbett - 11.JPG|Finished jewellery ==In popular culture==

"The Terror of Blue John Gap" is a short story by Sir Arthur Conan Doyle, which appeared in The Strand Magazine of 1910 and describes the experiences of a doctor who, while recuperating from tuberculosis on a Derbyshire farm, investigates mysterious goings-on in a cavern mined for Blue John. ==References==