RocksdayAmethyst vs Fluorite
How to tell them apart
The purple variety of quartz. Its color comes from traces of iron combined with natural radiation, and it often lines the inside of large volcanic geodes.
Photo: Gonzalo Devia, CC BY-SA 4.0
A colorful mineral that grows as cubes and splits into octahedrons. Many specimens glow under ultraviolet light — the word 'fluorescence' was coined from it.
Photo: CarlesMillan, CC BY-SA 3.0
Quick tests
- Scratch glass: Amethyst scratches glass (Mohs 7); Fluorite can't (Mohs 4).
Key differences
- Purple fluorite is much softer (4), forms cubes and splits cleanly along flat planes.
- Amethyst is harder (7), forms pointed six-sided crystals and has no cleavage.
Side by side
| Property | Amethyst | Fluorite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 7 | 4 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | Purple | Purple, Green, Blue, Yellow, Clear |
| Crystal system | Trigonal | Cubic |
| Specific gravity | 2.65 | 3.18 |
| Formula | SiO₂ | CaF₂ |
| Found in | Rio Grande do Sul, Brazil; Artigas, Uruguay | Cave-in-Rock, Illinois; Weardale, England |
Properties that are the same for both are grayed out.
How to identify amethyst
- Purple, often deepest at the crystal tips.
- Hardness 7 — scratches glass.
- Grows as clusters of pointed crystals inside geodes.
- No cleavage; breaks with a glassy, curved fracture.
Crystal system: Trigonal
Full Amethyst guide →How to identify fluorite
- Cubic crystals, often with color zoning in purple and green.
- Cleaves into eight-sided octahedral fragments.
- Hardness 4 — a knife scratches it, but a coin doesn't.
- Often glows blue or violet under a UV flashlight.
Crystal system: Cubic
Full Fluorite guide →More comparisons
Could you tell them apart from a blurry close-up?
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