Blue John

The name Blue John derives from French "bleu et jaune" (blue and yellow) characterising its colour. Purple-blue fluorite was mined in Castleton, Derbyshire, England including the famous Blue John Cavern.

A wide range of ornaments are manufactured from this colourful material.
General Information
A variety or type of:Fluorite
Chemical Formula
CaF
 
2
Michael O’Donoghue, Gems, Sixth Edition (2006)
Photos of natural/un-cut material from mindat.org
Blue John Treatments
Blue John can be lightened by a gentle heating to 100-150°C and is totally removed at 200-300°C. Irradiation can restore the color of overheated Blue John or produce blue and other colors in colorless materials; these colors may be stable to light or may fade, depending on the material.
Plastic, usually epoxy resin, is used to impregnate Blue John so that thinner-walled and more delicate objects can be fashioned from it. If heat is used to cure the resin, care must be taken that the color is not lost by heating, although it could be recovered by irradiation if the resin does not darken at the same time - Nassau (1984)
Physical Properties of Blue John
Mohs Hardness4 to 0
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Specific Gravity3.00 to 3.25
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Cleavage QualityPerfect
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Optical Properties of Blue John
Refractive Index1.432 to 1.436
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Optical CharacterIsotropic
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
Colour
Colour (General)Banded white/purple
Gemdat.org, Management Team (2012)
TransparencyTransparent,Translucent
Arthur Thomas, Gemstones (2009)
LustreVitreous
Arthur Thomas, Gemstones (2009)
Crystallography of Blue John
Crystal SystemIsometric
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
HabitCubic, massive
Arthur Thomas, Gemstones (2009)
Further Information
Mineral information:Blue John information at mindat.org
Significant Gem Localities
UK
 
  • England
    • Derbyshire
      • Castleton
[www.thamesvalleyminerals.com]
Fluorite: The Collector's Choice. Extra Lapis English No. 9
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