Gem library
Fluorite
The stone that gave fluorescence its name
Fluorite is the most polychrome mineral in the kingdom: violet, green, blue, yellow, pink, colourless, and often several colours zoned within a single cubic crystal. It also gave fluorescence its name, the phenomenon having been described on its specimens in 1852.
- Hardness (Mohs)
- 4
- Density
- 3.00 – 3.25
- Crystal system
- Cubic
- Refractive index
- 1.433 – 1.435
- Cleavage
- Perfect on {111}
Mineralogical identity
- Family
- Halides
- Species
- Fluorite
- Variety
- Fluorspar, Blue John, chlorophane
- Chemical formula
- CaF₂
- Crystal system
- Cubic
- Classification
- Collector's mineral
- Rarity
- Common
- Indicative price
- €0.50 – 15/ct
Etymology
From the Latin fluere, «to flow»: added to ores, it lowers their melting point. It gave its name to the phenomenon of fluorescence, described by George Stokes in 1852.
Physical & optical properties
- Hardness (Mohs)
- 4
- Density
- 3.00 – 3.25
- Refractive index
- 1.433 – 1.435
- Birefringence
- None
- Cleavage
- Perfect on {111} — four octahedral directions
- Fracture
- Conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- Strong
- Pleochroism
- None
- Colours
- purple, green, blue, yellow, pink, colourless, multicoloured
Formation, geology & history
The perfect cube and octahedral cleavage
Fluorite crystallises in the cubic system and forms cubes of remarkable geometric crispness, sometimes modified into octahedra or dodecahedra. Its cleavage is perfect along the four octahedral planes, which allows the corners of a cube to be removed to obtain regular octahedra — the famous fluorite octahedra sold in mineralogy are in fact often cleaved, not natural. That multiple cleavage, combined with a hardness of only 4, makes it a fragile mineral: it is scratched by a knife, chips at the slightest knock, and is entirely unsuited to rings. Its fracture is conchoidal and its edges blunt quickly.
Fluorescence, thermoluminescence and Blue John
Under ultraviolet light many fluorites glow an intense blue-violet, due to traces of europium and yttrium — it was on such specimens that Stokes described and named fluorescence. Certain varieties, called chlorophane, also emit light when heated (thermoluminescence) or rubbed. Blue John from Derbyshire, England, is a banded violet, blue and cream fluorite, worked since the eighteenth century in a single cavern in the world, Treak Cliff Cavern. Its output, a few hundred kilos a year, goes into vases and jewellery of extreme rarity.
A major industrial mineral
Beyond collecting, fluorite is the main source of fluorine: it serves as a flux in steelmaking and enters the manufacture of hydrofluoric acid, fluoropolymers such as PTFE, refrigerants and the fluoride in toothpaste. Optical fluorite, colourless and perfectly pure, is used to make very low dispersion lenses for high-end camera optics and telescopes — an application that exploits its exceptionally low refractive index.
How to recognise it
- Clean cubes or octahedraThe perfect cubic form, often with stepped interlocking cubes, is a major clue. No quartz, no amethyst crystallises as a cube.
- Scratched by a knifeHardness 4: a knife marks it without effort, while it does not scratch glass. This is the decisive test against amethyst (7).
- Blue-violet fluorescence under UVMost fluorites glow intensely under a 365 nm UV lamp. A quick and spectacular test.
Imitations, treatments & confusions
- Multicoloured cast glassBubbles, mould seams, no octahedral cleavage, hardness 5-6 (a knife does not scratch it).
- Confusion with amethystFrequent on translucent violet stones. Fluorite is scratched by a knife, forms cubes and fluoresces; amethyst scratches glass and grows as hexagonal prisms.
- Dyed or irradiated fluoriteSome pale fluorites are irradiated to deepen the violet. A stable treatment but one that must be declared; the tone is often too uniform.
Deposits & origins
China (largest world producer), Mexico, Spain (Asturias — colourless and violet fluorites of great quality), England (Derbyshire: Blue John, a unique banded variety), Germany, United States (Illinois), France (Puy-de-Dôme, Tarn, Le Burc), Morocco, Namibia
Care & precautions
- Brief rinsing in cold water only. Fluorite is slightly soluble and sensitive to acids.
- Never direct sunlight: many fluorites, especially violet and pink ones, fade irreversibly.
- Extremely sensitive to impact and temperature change: the fourfold cleavage makes it split. Store individually in cotton.
- Ultrasonic cleaners and steam are absolutely forbidden.
Traditions & crystal healing
- Chakras
- Third eye · crown
- Zodiac signs
- Pisces · Capricorn
- Element
- Air
Tradition nicknames it «the mental broom»: it is said to help sort ideas, structure a confused project and filter demands. It is readily placed near a workstation or a screen. Its fragility makes it an indoor stone, rarely worn on the body.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Fluorite or amethyst?
The knife test settles it in two seconds: fluorite is scratched by a knife (hardness 4), amethyst scratches glass (hardness 7). Fluorite also forms cubes and fluoresces under UV.
Can you wear a fluorite ring?
It is strongly discouraged: hardness 4 and perfect cleavage in four directions. A fluorite ring chips and splits within months of normal wear.
What is Blue John?
A banded violet, blue and cream fluorite, extracted from a single Derbyshire cavern since the eighteenth century. Annual output is measured in hundreds of kilos: one of the rarest ornamental stones in the world.