Fluorite, also called fluorspar, is calcium fluoride. It forms cube-shaped crystals in almost every color, most often violet, green, yellow or colorless, often several colors in one piece. With a Mohs hardness of 4 it is so soft that it marks exactly this step on the Mohs scale. Its name comes from the Latin fluere, to flow, because for centuries it has served as a flux when smelting ores, and in turn it gave fluorescence its name. One of the best known German localities is the Clara Mine near Oberwolfach in the Black Forest.
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How fluorite forms, using the example of the Clara Mine
The Clara Mine lies in the upper Rankach valley near Oberwolfach in the Ortenau district, in the middle of the Central Black Forest. Mining has taken place here since 1898, and today it is the last active one of what were once many mines in the Black Forest. It works a thick vein that consists on average of 55 to 90 percent fluorite, along with baryte and some quartz. The following images show how this vein formed.
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Hot solutions rise
Here the basement rock of the Black Forest consists of gneiss, a very old rock transformed by pressure and heat. It is crossed by deep fault zones along which the earth's crust has moved. Between the Jurassic and the Tertiary, hot, watery solutions circulated in these fissures. They came from great depth, had dissolved fluorine from the rocks on their way and rose up through the fissures. According to current understanding, fluorite precipitated mainly where these solutions mixed with water that seeped in from above and brought calcium with it. Calcium and fluorine form calcium fluoride, which is barely soluble: it crystallizes out.
The fissure fills up
The fluorite grew inwards from the walls of the fissure, layer by layer. Because the composition of the solutions kept changing, the layers change color, sometimes violet, sometimes green, with white bands of quartz or baryte in between. This is how banded vein fluorite forms, which looks like the growth rings of a tree in a cut. If a cavity remained open in the middle, free-standing cubes could grow into it, often together with tabular baryte crystals. Such druses are the finds collectors look for.
Cubes with color zones
Pure fluorite is colorless. It gets its colors from traces of foreign elements such as rare earths and from tiny defects in the crystal lattice, created for example by natural radiation. Because conditions changed during growth, many cubes are colored in zones inside: each layer shows a growth spurt, and at the edges the zones can be seen as fine lines parallel to the surface. On top of that comes a property that is decisive for processing: fluorite cleaves perfectly in four directions, parallel to the faces of an octahedron. If you knock a corner off a cube, a smooth, triangular face appears, and if you keep cleaving, you end up with an octahedron.
Today at the Clara Mine
Since 1898 the Clara Mine has produced more than two million tonnes of fluorspar and more than three million tonnes of baryte. It is also one of the most mineral-rich localities in the world: over 375 different minerals have been described from here, 15 of them found here for the first time. Every year thousands of visitors search the mine dump for minerals, and violet or green fluorite is among the most common finds.
A special case: the stinking fluorite of Wölsendorf
In the Maria Mine near Wölsendorf in the Upper Palatinate, an almost black-violet variety occurred that gives off a pungent smell when struck. For almost 200 years people argued about where the smell came from. In 2012 researchers from TU Munich and LMU Munich proved that tiny inclusions of uranium constantly split the fluorite with their radiation. This creates elemental fluorine, which stays trapped in minute cavities and is released when the stone breaks. It was the first direct proof of elemental fluorine in nature. This stinking fluorite, also called antozonite, shows particularly clearly how radiation can color fluorite dark.
How to recognize it
Typical are cube-shaped crystals with a vitreous luster, often in strong violet or green and with color zones running parallel to the cube faces. With a Mohs hardness of 4, fluorite is much softer than quartz and can be scratched with a pocket knife. It feels heavy for its size, with a density of about 3.2 g/cm³. Broken corners show smooth, triangular cleavage faces, often in small steps. Many pieces glow blue to bluish violet under UV light.
Violet fluorite is most often mistaken for amethyst even though amethyst is much harder, forms six-sided points instead of cubes and has no cleavage. Calcite is even softer, cleaves into oblique rhombohedra and fizzes with dilute hydrochloric acid. Colored glass has neither cleavage faces nor color zones.
Where it occurs
Fluorite occurs all over the world. Today the largest quantities come from China and Mexico; also famous among collectors are the deposits in England, such as the banded Blue John from Derbyshire. In Germany the Black Forest is the most important area. Besides the Clara Mine, the Käfersteige mine near Pforzheim was located here and operated from 1935 to 1996. Its vein is considered probably the largest fluorspar deposit in Europe, and since 2025 a procedure has been under way to mine it again. Other well known deposits are in the Upper Palatinate around Wölsendorf and in the Thuringian Forest.
Fluorite also occurs in the southern Black Forest, not far from the Markgräflerland, in the old ore veins, for example in the district around Sulzburg, where it is found together with baryte, quartz and amethyst in the same veins.
From flux to lens
Miners and smelters used fluorite as a flux for centuries: added to the ore, it makes the slag more fluid and lowers the melting temperature. That is where the name fluorspar comes from. Today fluorite is above all the raw material of fluorine chemistry; hydrofluoric acid is made from it, and from that a long series of fluorine compounds. Clear, synthetically grown fluorite crystals are also used for high-quality camera and telescope lenses, because they correct color fringes particularly well.
Profile
| Mineralogy | |
| Classification | fluorite, called fluorspar in mining; varieties include stinking fluorite (antozonite) and Blue John |
| Locality in this example | Clara Mine, Rankach valley near Oberwolfach, Ortenau district, Baden-Württemberg; vein in the gneiss of the Black Forest basement |
| Chemical formula | CaF₂ (calcium fluoride) |
| Mineral class | halides |
| Crystal system | cubic; mostly cubes, more rarely octahedra |
| Mohs hardness | 4 (reference mineral of the Mohs scale) |
| Density | about 3.2 g/cm³ |
| Color | colorless, violet, green, yellow, blue, more rarely pink, brown or black-violet; often color zoned |
| Streak | white |
| Luster | vitreous |
| Transparency | transparent to translucent |
| Fracture | conchoidal to splintery |
| Cleavage | perfect in four directions (octahedral) |
| Special feature | fluorescence under UV light, usually blue to bluish violet; the term fluorescence is named after fluorite |
| Processing | |
| Difficulty | demanding despite its low hardness, because of the perfect cleavage |
| Sawing | easy to saw; existing cleavage cracks can open up during the cut |
| Grinding & polishing | a flat polish is difficult: small flakes break out along the cleavage planes and leave dull spots |
| Drilling | possible, with little pressure; chipping at the exit point is more likely than with quartz |
| Typical risks | flake-like breakouts, cleavage cracks, scratches due to the low hardness, cracks from rapid changes in temperature |
| Suitable for | cut pieces and flat polish of banded vein fluorite, slices, pendants and earrings; too soft for rings |
Processing notes
Fluorite is soft and can therefore be sawn and ground quickly. It only becomes difficult at the last step: a truly flat polish on fluorite is one of the trickiest tasks in the workshop. The reason is its perfect cleavage in four directions. During grinding and polishing, small flake-like platelets keep coming loose from the surface along the cleavage planes. Where such a flake has broken out, a dull spot remains, and the surface has to be built up again from a coarser grit.
With fluorite it helps to work with little pressure and to go through the grits more slowly. Even so, individual dull spots cannot be ruled out completely, especially in pieces with many cleavage cracks. It is also important to avoid sudden temperature changes, because fluorite reacts sensitively to rapid changes between warm and cold.
Banded fluorite from veins is particularly suitable for cut pieces and flat polishes, because the color layers look their best on a polished surface. As jewelry it is suitable for pendants and earrings; on rings it would quickly get scratched because of its low hardness. For sawing, fluorite falls under the standard rate of €0.15 per cm².
- Sawing: cut pieces, slices, splitting vein pieces
- Grinding and polishing: from fine grinding to polish
- Drilling: straight on and with little pressure
- Turning it into jewelry: from find to pendant
You can calculate the guide price for your piece in the stone processing calculator.
Frequently asked questions
Why is fluorite also called fluorspar?
Because it was used as a flux when smelting ores. It makes the slag more fluid, so it flows better. The name fluorite also goes back to the Latin fluere, to flow.
Why does fluorite glow under UV light?
Traces of foreign elements in the crystal absorb the invisible UV light and give it off again as visible light, usually blue to bluish violet. The phenomenon was described on fluorite in the 19th century and named fluorescence after it. Not every piece glows; it depends on the locality.
Can fluorite be polished to a high gloss?
Yes, but it is difficult. Because of the perfect cleavage, small flakes easily break out of the surface during polishing and leave dull spots. With a lot of patience and little pressure, a beautiful luster can still be achieved.
Is fluorite suitable for jewelry?
Yes for pendants and earrings, if it is treated with some care. With a Mohs hardness of 4 it quickly gets scratched on rings or bracelets, and hard knocks can make it break along the cleavage planes.
How do I tell violet fluorite from amethyst?
Most easily by shape and hardness: fluorite forms cubes and can be scratched with a knife, amethyst forms six-sided points and is harder than steel. Broken corners with smooth, triangular faces point to fluorite.
Sources
- Flussspat im Schwarzwald, Grube Clara. In: LGRBwissen, Landesamt für Geologie, Rohstoffe und Bergbau Baden-Württemberg.
- Grube Clara. In: Wikipedia, Die freie Enzyklopädie.
- Fluorit. In: chemie.de, Lexikon.
- Jahrhundertealter Streit um stinkendes Mineral aus der Oberpfalz geklärt. Press release of TU Munich, 2012.
- Wiederaufnahme der Grube Käfersteige, summary of the environmental impact report. Deutsche Flussspat GmbH, City of Pforzheim, 2025.