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.
A wide range of ornaments are manufactured from this colourful material.
| General Information | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| A variety or type of: | Fluorite | ||||||||
| Chemical Formula |
| ||||||||
| 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 Hardness | 4, Blue Chart Gem Identification (2010) | ||||||||
| Specific Gravity | 3.00 to 3.25, Blue Chart Gem Identification (2010) | ||||||||
| Cleavage Quality | Perfect, Blue Chart Gem Identification (2010) | ||||||||
| Optical Properties of Blue John | |||||||||
| Refractive Index | 1.432 to 1.436, Blue Chart Gem Identification (2010) | ||||||||
| Optical Character | Isotropic, Gemmological Tables (2004) | ||||||||
| Colour | |||||||||
| Colour (General) | Banded white/purple, Management Team (2012) | ||||||||
| Transparency | Transparent,Translucent, Gemstones (2009) | ||||||||
| Lustre | Vitreous, Gemstones (2009) | ||||||||
| Crystallography of Blue John | |||||||||
| Crystal System | Isometric, Blue Chart Gem Identification (2010) | ||||||||
| Habit | Cubic, massive, Gemstones (2009) | ||||||||
| Further Information | |||||||||
| Mineral information: | Blue John information at mindat.org | ||||||||
| Significant Gem Localities | |||||||||
| |||||||||