Fluorite
Calcium fluoride (CaF₂)
A soft, colorful calcium fluoride mineral famous for cubic crystals, perfect octahedral cleavage, and fluorescence under UV light.
- Mohs hardness
- 4
- Color
- Purple, green, blue, yellow, colorless, multicolor
- Type
- mineral
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Overview
Fluorite is calcium fluoride (CaF₂), one of the most colorful minerals in the world—it can be purple, green, blue, yellow, pink, colorless, or banded in multiple colors within a single crystal. It is the mineral that gave the phenomenon of fluorescence its name, often glowing blue or purple under ultraviolet light.
Fluorite commonly forms beautiful cubic crystals and has perfect octahedral cleavage, meaning it can be cleaved into eight-sided diamond shapes. It is relatively soft (hardness 4), defining that point on the Mohs scale.
Its beauty, abundance, and importance as an industrial flux make it a favorite of collectors and chemists alike.
Formation & geology
Fluorite forms chiefly in hydrothermal veins, often alongside minerals like galena, sphalerite, barite, and quartz, where fluorine-bearing fluids deposit it as cavities and fractures fill. It also occurs in some limestones, dolomites, and as a late-stage mineral in pegmatites.
Color zoning arises from changing trace elements and natural radiation during growth, producing the famous banded "rainbow" fluorite.
Major localities include England (Cumbria, the Derbyshire 'Blue John' variety), China, Mexico, the USA (Illinois), and Spain.
How to identify it
- Hardness: 4 on the Mohs scale—it can be scratched by a steel knife and won't scratch glass (distinguishes it from quartz).
- Cleavage: Perfect octahedral cleavage in four directions; broken pieces form eight-sided shapes.
- Crystal habit: Cubes and octahedra; often color-zoned.
- Fluorescence: Many specimens glow under UV light.
Look-alikes: Quartz and amethyst are much harder (7) and lack cleavage. Calcite is softer (3) and fizzes in acid; fluorite does not react with weak acid. Color plus cubic habit plus moderate softness is diagnostic.
Uses & significance
Fluorite is industrially vital as 'fluorspar,' the primary source of fluorine. Metallurgical-grade fluorite is used as a flux in steelmaking; chemical grade makes hydrofluoric acid and fluorochemicals; optical-grade clear fluorite is used in high-quality camera and microscope lenses because of its low dispersion.
As a collector and lapidary stone, fine cubic crystals and carvings (like English Blue John) are prized, though its softness and cleavage make faceted fluorite delicate.
Metaphysically it is associated with focus and mental clarity—cultural belief, not science.
Frequently asked questions
Is fluorite hard enough for jewelry?
Not really—at hardness 4 with perfect cleavage, fluorite scratches and chips easily, so it's best in pendants or earrings rather than rings.
Why does fluorite glow under UV light?
Trace impurities like rare-earth elements absorb ultraviolet light and re-emit it as visible light; this effect is literally named after fluorite.
What is rainbow fluorite?
Rainbow fluorite is a single specimen with bands of different colors (often purple, green, and blue) caused by changing trace elements during crystal growth.
How can you tell fluorite from quartz or amethyst?
Fluorite is much softer (4 vs. 7) and has perfect octahedral cleavage, while quartz is harder and fractures conchoidally with no cleavage.
Fluorite guides
In-depth guides for identifying, valuing, and understanding Fluorite.
Fluorite identified by the community
Real specimens identified with Rock Identifier.
