Fluorite: Gemstone Information
Fluorite, CaF2 , for which fluorspar is an old name, forms fine transpar-ent crystals in a range of colours which include colourless, yellow,brown, green, blue, violet and pink. The green may resemble that of emerald in specimens found near Otjiwarongo, Namibia.Blue John is the traditional name for a highly characteristic massive banded translucent white/blue/violet/purple variety of fluorite found in the Castleton area, Derbyshire, England. Fluorite occurs as cubic crystals with an octahedral perfect and easy cleavage which often gives rise to the mistaken impression that the mineral forms octahedra in preference to cubes. Interpenetrant twinning is common and dodecahedra and combinations can be found as well as a number of other forms.Virtually all fluorite octahedra found on sale have been cleaved by dealers. Fluorite has a hardness of 4 and SG usually near 3.18; the RI is 1.432–1.434.Under LWUV blue, violet, red, green and yellow responses have been reported. It may show thermoluminescence or triboluminescence and some specimens phosphoresce.
Blue John shows no response to energies above those of visible light. Fluorite is named for its low melting point.Fluorite occurs in low-to-high temperature hydrothermal veins, granites and granite pegmatites. Details of the fluorite occurrences in Derbyshire,England, can be found in different works by Trevor D. Ford including Derbyshire Blue John (2000: 1873775199) and the fluorite deposits of the Northern Pennine Orefield, England, are covered by successive editions of Geology of the Northern Pennine Orefield, the latest being volumes 1 (1990) and 2 (1985) by K.C. Dunham in the series Economic Memoirs of the British Geological Survey. The Friends of Killhope’s Fluorspar in the North Pennines (2001) is a useful guide to the fluorite of that area as are British Mining 47 and 57, dealing with Alston Moor (1993) and the Weardale Mines (1996), the publishers of the British Mining series are the Northern Mine Research Society.Most fine crystals of fluorite are not faceted (though faceted stones can be very beautiful) as fine groups sell for high prices.
Examples of deep pink to red fluorite from the Chamonix area of the European Alps were exhibited at the Natural History Museum in London some years ago and attracted great interest. Fluorite showing smoky-brown in daylight and mauve in incandescent light has been reported from India.Gem-quality fluorite from deposits in Hardin and Pope counties,Illinois, USA, is discussed in detail by Bastin in Bulletin no. 58 (1931) of the State Geological Survey of Illinois; the deposits extend into Crittenden county, Kentucky. The Illinois mines are centred in the Rosiclare district and also at Cave-in-Rock. Crystals show a wide variety of colours with characteristic zoning parallel to crystal faces.The cause of the violet colour in some fluorite has been ascribed to Mie scattering from microcrystals of calcium metal; this is cited by Nassau in The Physics and Chemistry of Colour (2000).
The blue colour arises from the presence of an F-centre. Colourless fluorite can be irradiated with energetic radiation to give F-centres, named from the German Farbe (colour).The specimen will turn purple. Nassau (2000) gives an explanation of the mechanisms involved.The blue luminescence seen in fluorite, on the other hand, is believed to arise from the presence of Eu2 or an associated colour centre. Bill, in Physics and Chemistry of Minerals 3, 117–31 (1978) cited by Nassau (op. cit.),states that a colour centre combination of Y3+ with a fluorine vacancy can also give a blue colour and that a combination of this with Ce gives a yellowish green. O 3-substituting for F-gives yellow and Y3+ + O2 3 a pink.In green fluorite, weak bands have been reported at 634, 610, 582 and 445 nm, and there is also a strong broad band at 427 nm which may be seen in sizeable pieces.A part of the extraLapis series (no. 4, 1993, ISBN 3921656273) includes valuable papers and beautiful photographs.