Blue John (mineral)

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.

[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]: 39  Elsewhere, blue fluorspar is known in the Ardennes region of Belgium; the Cave-in-Rock area of Illinois in the United States;[1]: 39  and at various localities in Mexico and China.

Recently discovered deposits in China have produced fluorite with colouring and banding similar to the classic Blue John stone.

[citation needed] Roman writers, such as Pliny the Elder, refer to a soft ornamental rock which they called murrhine, out of which drinking vessels were carved.

[1]: 53  However, two cups have been found among Roman grave-goods near the Turkish/Syrian border, on the former Persian-Roman trade route, which were made from blue-yellow banded fluorspar presumably obtained from the Persian source mentioned by Pliny.

Today, production is probably around half a ton per annum,[1]: 42  and the raw Blue John produced is kept within Castleton where small articles, mainly jewellery, are worked and sold.

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.

They may be made into rough cuboids or cylinders ("rough-outs"), for turning as bowls and vases, or flat slices, for making jewellery.

A final high polish is added using putty powder (finely crushed tin dioxide) applied with a moist piece of felt.

For making jewellery, thin slices are marked out and cut into shapes such as circles or ovals, then finished on a grinding wheel.