|Wedge of "Blue John", displaying beautiful banding. Photo courtesy of www.mineral-forum.com|
A banded variety of fluorite (calcium fluoride), Blue John is supposedly named as such because French prospectors from the reign of Louis XVI imported the material and described it by its colors: blue and yellow (bleu, jaune). This story is not yet substantiated by records in France, but it does aptly illustrate the kind of distinct color zoning and banding that is seen in this stone, ranging anywhere from purplish blue, to yellowish cream. This particular variety of fluorite has only been found in Derbyshire, central England. Other fluorite can also be found mainly in China and the U.S.A.
|A fluorite rough specimen. Photo courtesy of www.mtgms.org|
|Regency-era "Blue John" vases, circa 1810-1820. Photo courtesy of www.coulborn.com|
Apart from its basic properties, fluorite is most interesting because of its ability to fluoresce when exposed to UV lighting. For those who have been to laser tag (the notoriously dark rooms that are lit with certain lights making certain items fluoresce) are actually similar to the effect seen in fluorite. This light excites the electrons within the material to a point where it must release energy in order to stabilize itself. It does so by emitting a luminescent "glow". While they are not the only gemstones that have this ability, fluorite generally has a very pronounced reaction to it. Some attribute this to the REE (Rare Earth Elements) within the material.
|Tumbled fluorite before and during exposure to UV light. Excellent example of fluorescence. Photo courtesy of www.geology.com|
I don't know about you, but when hearing about new discoveries like this, I have the urge to go out and explore. You never know what you'll find!