Fluorite Identification Guide
How to identify fluorite by its cubic crystals, perfect octahedral cleavage, hardness of 4, vivid colors, and fluorescence, with tests to separate it from quartz and calcite.
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What Fluorite Looks Like
Fluorite is calcium fluoride (CaF2), one of the most colorful and recognizable minerals. It crystallizes in the isometric (cubic) system.
- Color: famously variable — purple, green, blue, yellow, colorless, pink, and often color-zoned or banded in a single crystal.
- Luster: vitreous (glassy).
- Transparency: transparent to translucent.
- Crystal habit: classic interpenetrating or stacked cubes; also octahedra and massive/granular forms. Penetration twins are common.
Step-by-Step Field-ID Checklist
- Look for cubes. Well-formed cube crystals are an immediate flag.
- Check the hardness. Fluorite defines Mohs 4 — a steel knife scratches it easily, but it does not scratch glass.
- Find cleavage. Fluorite has perfect octahedral cleavage in four directions; broken pieces often show triangular cleavage faces and corner-cut octahedral fragments.
- Note color zoning and the glassy luster.
- Test fluorescence under UV light.
Key Diagnostic Tests
- Mohs hardness: 4 (index mineral). Scratched by a knife and even by an iron nail; will NOT scratch glass — this rules out quartz immediately.
- Streak: white.
- Cleavage: perfect, octahedral, four directions — diagnostic. Cleaved corners produce neat octahedral shapes.
- Density: ~3.0–3.2 g/cm3, noticeably heavier than quartz.
- Acid: no fizz with cold dilute HCl (separates it from calcite, which fizzes vigorously).
- Fluorescence: many specimens glow blue, violet, or other colors under UV (the phenomenon is named after fluorite); some show thermoluminescence.
Common Look-Alikes and How to Tell Them Apart
- Quartz/amethyst: also purple/colorless and glassy, but quartz is hardness 7 (scratches glass), has NO cleavage (conchoidal fracture), and grows as hexagonal prisms with points, not cubes. The hardness test is decisive — a knife scratches fluorite, not quartz.
- Calcite: softer (3) and fizzes in acid; fluorite does not fizz. Calcite has rhombohedral cleavage, fluorite octahedral.
- Glass: no cleavage, no crystal faces, will not show octahedral cleavage fragments, and is usually harder.
- Topaz: much harder (8) with one direction of cleavage; topaz scratches glass, fluorite does not.
- Apatite: hardness 5, hexagonal crystals, poor cleavage; apatite is slightly harder and forms prisms, while fluorite forms cubes with easy octahedral cleavage.
The killer combination is cubic crystals + hardness 4 (knife scratches it, it cannot scratch glass) + perfect octahedral cleavage + no acid fizz + frequent fluorescence.
Where Fluorite Is Found
Fluorite is a common hydrothermal vein and gangue mineral, often with galena, sphalerite, barite, and quartz. Classic localities include England (Weardale and the Cumbria/Derbyshire 'Blue John' banded fluorite), the Illinois-Kentucky fluorspar district (USA), China, Spain, Mexico, and Switzerland (gem octahedra in Alpine clefts). It also occurs in limestones and pegmatites.
Frequently asked questions
How can you tell if a crystal is fluorite?
Look for glassy, often colorful cube crystals with a hardness of 4 (a steel knife scratches it easily, and it cannot scratch glass), perfect octahedral cleavage that produces triangular and octahedral fragments, no fizz in acid, and frequent glowing fluorescence under UV light.
What is the difference between fluorite and quartz?
Fluorite is hardness 4 and is scratched by a knife, has perfect octahedral cleavage, and forms cubes. Quartz is hardness 7, scratches glass, has no cleavage (it fractures conchoidally), and forms six-sided prisms with points. The scratch test alone separates them.
Does fluorite glow under UV light?
Many fluorite specimens fluoresce blue, violet, or other colors under ultraviolet light, and the word 'fluorescence' was actually named after fluorite. Not every specimen glows, but a glowing reaction supports the identification.
How do I tell fluorite from calcite?
Calcite fizzes vigorously in dilute acid and has rhombohedral cleavage; fluorite does not fizz and has octahedral cleavage. Fluorite (hardness 4) is also slightly harder than calcite (hardness 3).
What colors does fluorite come in?
Fluorite is one of the most colorful minerals, occurring in purple, green, blue, yellow, pink, and colorless, and often shows multiple bands or zones of color within a single crystal.
Fluorite identified by the community
Recent Fluorite specimens identified with Rock Identifier.
