The Mohs 4 reference mineral. Cubes, perfect octahedral cleavage, and the mineral that gave fluorescence its name.
China, Mexico, England (Weardale and Castleton), Illinois, Namibia5 field tests
Fluorite is calcium fluoride, and it is the reference point for 4 on the Mohs scale — which is the single most useful fact about it, because a steel knife cuts it easily and that separates it from almost everything it is mistaken for. Purple fluorite gets sold as amethyst, green fluorite as emerald, and both fail a knife test in one second.
Its second signature is cleavage. Fluorite cleaves perfectly in four directions, along the faces of an octahedron, which means a broken piece shows flat triangular mirror faces and a struck cube tends to lose its corners in neat octahedral chips. Nothing in the quartz family does this — quartz has no cleavage at all.
The third is the property that carries its name. Fluorite fluoresces under ultraviolet light, usually a strong blue-violet, and the phenomenon was named after the mineral rather than the other way round. A cheap UV torch makes this an instant and rather satisfying confirmation.
Fluorite is unusually easy to identify because three of its properties are distinctive and all three are free to test. Hardness 4, four-directional octahedral cleavage, and fluorescence under UV: any two of those together are conclusive.
It comes in essentially every colour there is, often several in one crystal in sharply bounded zones, which is why colour is useless here and why the mineral is so widely mistaken for more valuable stones.
| Property | Fluorite | What it rules out |
|---|---|---|
| Mohs hardness | 4 — the scale's own reference | Quartz and amethyst (7); emerald (7.5–8) |
| Cleavage | Perfect, four directions, octahedral | Quartz, which has none |
| Specific gravity | 3.0 – 3.25 | Quartz 2.65 — fluorite feels heavier |
| Fluorescence under UV | Usually strong blue-violet | Amethyst, which is inert |
| Crystal habit | Cubes, octahedra, interpenetrant twins | Trigonal quartz prisms |
| Colour | Purple, green, blue, yellow, colourless, pink — often zoned | Nothing. Colour proves nothing here |
| Streak | White | — |
| Reaction to acid | None to weak acid | Calcite, which effervesces |
A purple stone that a knife marks, that breaks on flat triangular faces and that glows under UV is fluorite, not amethyst. That comparison is the most common reason anyone looks it up.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Fluorite is the reference mineral for hardness 4, so a steel blade marks it without effort and a copper coin will just begin to. This is the whole separation from quartz, amethyst, emerald and beryl, all of which are 7 or above and untouched by steel. Do it on a hidden face — fluorite is a soft mineral and scratches are permanent.
Fluorite cleaves perfectly along four octahedral directions, so damaged crystals lose their corners as neat triangular flats and broken fragments show mirror-bright planes meeting at consistent angles. Quartz has no cleavage and always breaks in curved conchoidal shells. One glance at a chipped corner separates them.
Most fluorite fluoresces under longwave ultraviolet, typically a strong blue-violet, and some material from Weardale in England glows spectacularly. Amethyst, quartz and glass are essentially inert. A UV torch costs very little and turns this into a two-second check.
Fluorite crystallises in the cubic system and forms excellent cubes, often with faces stepped or with smaller cubes perched on larger ones, and frequently with sharply bounded colour zones parallel to the cube faces — a purple cube with a green core, or bands following the crystal's geometry. That zoning pattern is characteristic and no quartz reproduces it.
At 3.0–3.25 fluorite is noticeably denser than quartz at 2.65. It is not a dramatic difference, but with a piece of each in hand it is real, and it supports the harder evidence.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
Multiple colour zones in one piece, following the crystal's own geometry.
Read within the checks aboveSpecimen figures rather than gem figures, for material in honest condition. Most finds are worth very little, and saying so plainly is more useful than encouraging you.
| Form | What it typically fetches | Note |
|---|---|---|
| Tumbled or polished fluorite | $2 – 12 | Abundant |
| Cubic crystal cluster, China | $15 – 150 | Colour, lustre, undamaged edges |
| Fine English fluorite, Weardale | $100 – 1,500+ | Locality premium and fluorescence |
| Blue John, small modern turned piece | $40 – 300 | A near-exhausted single source |
| Antique Blue John vase or urn | Four to five figures | Antiques market, not mineral market |
Fluorite is soft and cleaves easily, so undamaged crystal edges are what specimen value turns on. English material carries a substantial locality premium.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.