Gem Focus – June 2021

Posted on June 7, 2021 by Çiğdem Lüle, PhD, FGA, GIA GG, DGA

Fluorite: Of fluorescence and fluorine.

Brightly colored crystals of fluorite, a.k.a., fluorspar, in large sizes have been a prized possession of mineral collectors for hundreds of years. This calcium fluoride mineral was first named by Carlo Antonio Galeani Napione in the late 18th century. He was inspired by the Latin word “fluere” meaning “to flow” because the mineral is used as a flux in iron smelting for industrial purposes. Later the term fluorescence and the name of element fluorine are derived from fluorite. There are numerous countries in the world that produce fluorite as it is also a major ore mineral for element fluorine. Each locality has its special characteristic when it comes to collectable mineral specimens. USA is one of the major producers of fluorite. It is also the official state mineral of Illinois.

Faceted green fluorite from New Hampshire, US. 237ct. Sold for $16,250 in 2013.
Courtesy of Heritage Auctions.

Fluorite forms in the cubic system with several different habits. If the crystals are transparent and fully formed, they are perfect candidates as display specimens. Fluorite’s four directional perfect cleavage and Mohs’ hardness of 4 are the major contributors to its low durability. Despite that, transparent and vividly colored crystals are popular amongst collectors. It also comes in almost every color of the spectrum and multicolored examples are not uncommon. Color zoning is not only limited to single crystal specimens but also seen in aggregate fluorite formations that are typically sought after for carvings. In fact, the world famous “Blue John Mine” in Derbyshire, England, UK, has been producing the unique purple-blue-yellow banded fluorite since the 18th century. Although similar colored and patterned fluorites were later discovered in different countries, notably in China, Blue John from England has a devoted group of collectors for its history. Exquisite and large ornamental objects carved from Blue John specimens are particularly well appreciated in local and national museums in England as British heritage.

Gemologists would easily separate fluorite from similar looking minerals by its low refractive index of 1.43. Its specific gravity is 3.18 which is easily felt when handled. The only phenomenon known to fluorite is color change and it is extremely rare. Since most banded and translucent fluorites are fashioned as beads, the weight of long strands and low hardness should be carefully considered during stringing and use. The faceted examples are mostly kept in collections unmounted but if they are mounted in jewelry, it is wise to keep them as earrings and pendants since even a small blow would split the stones alongside their cleavage planes. The same situation must be taken into account and such jewelry should not be put in ultrasonic cleaners.

Fluorite on baryte, sphalerite, and calcite from Elmwood Mine, Tennessee, US. 10.5 x 9.75 x 6 inches. Sold for $125,000 in 2013. Courtesy of Heritage Auctions

Fluorite is not known to be synthesized as a gemstone, nor is treatment common. It certainly would not take any heat, however, some irradiated examples from New Hampshire are reported. Pricing of fluorite varies immensely from a few dollars a strand for commercial grade beads to thousands of dollars for rare and large faceted gems. Most commonly traded sizes are 3 to 5 carats for cut stones in the middle market and if they are transparent with vivid colors, per carat price might be as high as a few hundred dollars retail.

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