halides
Fluorite
CaF2
- Also known as
- Blue JohnfluorsparDerbyshire spar
- Crystal system
- cubic
- Hardness
- 4 on the Mohs scale
- Lustre
- vitreous
- Colour
- Purple, green, blue, yellow, colourless, often banded in one crystal
- Where it forms
- In hydrothermal veins, often with lead and zinc ores, and in some limestones.
- Chiefly from
- Weardale, EnglandIllinois, United StatesHunan, ChinaNamibia
Why it is that colour
Colour centres from natural irradiation and from rare earth impurities. Fluorite is unusually good at accommodating trace elements, which is why a single crystal can be banded through several colours.
Safety
Safe to handle. It is a fluoride, and heating or treating it with acid releases hydrogen fluoride, which is severely corrosive — a laboratory and industrial hazard rather than a domestic one. Soft enough at hardness 4 to abrade easily, and not to be put in water for drinking.
Where it comes from
China supplies the majority of world fluorspar. Specimen material comes largely from China, Namibia, Mexico and the historic English fields. Fluorspar is on several countries’ critical minerals lists because it is the sole practical source of fluorine.
Historical use
Gives its name to fluorescence, because the phenomenon was first described in it by George Stokes in 1852. It is also the source of essentially all industrial fluorine — for refrigerants, for Teflon, for aluminium smelting, and for the fluoride in toothpaste. Optical-grade fluorite is still used in high-end camera lenses because of its very low dispersion.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Blue John from Derbyshire was carved into vases and urns in the eighteenth century and has a genuine English decorative history, but no symbolic tradition. The modern reputation for mental clarity and focus is New Age, and appears to be a pun on fluorite being used in optics.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Fluorite gave its name to fluorescence, which is a nice case of a mineral naming a phenomenon rather than the other way round. Stokes noticed that certain fluorite from Weardale glowed blue in ultraviolet light and coined the term in 1852.
Blue John deserves its own note. It is a banded blue-purple fluorite found in exactly one place — Treak Cliff in Derbyshire — and worked since the eighteenth century for ornamental vases, several of which are at Chatsworth. Very little is extracted now, a few hundred kilogrammes a year, and the deposit is effectively finite. It is one of the genuinely rare stones in this collection, and it carries no metaphysical reputation at all.
Identity checked against IMA-CNMNC master list, June 2025.