How to tell if fluorite is real
A fluorite crystal is hardness 4, density 3.175 to 3.184, cubic. Of 176 Handbook sheets only fluorite passes all three. Five specimen checks, in order.
To tell if fluorite is real, check three numbers in order: hardness 4, so a steel point marks it; measured density 3.175 to 3.184, so a 20.00 g piece weighs 13.70 to 13.72 g in water; and cubic crystals with flat octahedral cleavage. Of 176 Handbook sheets, only fluorite passes all three. Then look for coatings or dye.
In short
- The three-number filter. Of the 176 Handbook of Mineralogy sheets in this site's reference set, 168 print a numeric hardness. 28 of those span hardness 4, only fluorite and magnesite also overlap a density of 3.0 to 3.25, and only fluorite is cubic. A piece that passes all three checks is behaving like fluorite and like nothing else in the set.
- Day one needs no tools. Look at the corners and any broken edge. Flat, bright, triangular breaks are fluorite's octahedral cleavage; the corner test on why did my fluorite break shows the geometry. A piece with no flat break anywhere, only curved glassy surfaces, needs the next checks.
- Weigh it in water before you scratch it. Weighing does no damage. A clean 20.00 g fluorite weighs 13.70 to 13.72 g suspended in water; quartz, the usual confusion for purple fluorite, weighs 12.45 g, and calcite 12.62 g.
- Real fluorite can still be treated. The International Gem Society lists heating and irradiation for faceted fluorite and coating, dyeing and plastic impregnation for decorative objects. Treated fluorite is real fluorite that needs a disclosure, not a fake.
- The finish state is consistent with fluorite, not proven fluorite. Proof is a laboratory method: a GIA lab note identified the fluorite under a plastic coating by Raman spectroscopy. The identification guides are indexed at the collecting guides.
| Check | Fluorite | Quartz (incl. amethyst) | Calcite | Does it choose? |
|---|---|---|---|---|
| Hardness (Mohs) | 4 | 7 | 3 | Yes: steel marks fluorite and calcite, not quartz |
| Density, measured | 3.175 to 3.184 (IGS massive 3.0 to 3.25) | 2.65; 2.59 to 2.63 massive | 2.7102 | Yes: floor 3.0 |
| 20.00 g piece, weight in water | 13.70 to 13.72 g (13.33 to 13.85 g massive) | 12.45 g | 12.62 g | Yes on a 0.01 g scale: 1.08 g apart at the sheet density, 0.71 g at the 3.0 floor |
| Crystal system | Cubic | Hexagonal | Hexagonal | Yes, when crystals are present |
| Cleavage | {111} perfect: octahedral | Rarely observable; conchoidal fracture | {10-11} perfect: rhombohedral | Yes, on a broken edge |
| Refractive index | n = 1.433 to 1.448, isotropic | ω 1.544, ε 1.553 | ω 1.658, ε 1.486 | Yes, on a refractometer |
| Colour as recorded | Colorless, white, purple, blue, green, yellow, orange; red, pink, brown, bluish black; commonly zoned | Colorless, white; bluish violet among the colours from chemical or particulate inclusions; zoned or mottled | Colorless or white, also gray, yellow, green, many other colours from inclusions | No |
| Streak | White | White | White | No |
How to tell if fluorite is real: the five checks, in order
Real fluorite passes three numbers, and the order of the checks is set by what each one costs the specimen. The Handbook of Mineralogy prints fluorite as CaF2, cubic, hardness 4, measured density 3.175 to 3.184 (to 3.56 if high in rare-earth elements), with a perfect {111} cleavage and a white streak. The sequence below starts with the free, harmless checks and ends with the ones that leave a mark.
- Look (day one, no tools). Cubes and octahedra, flat triangular breaks across corners, and colour in zones that follow the crystal faces. The sheet records fluorite's colour as commonly zoned, so bands of purple, green and colourless are not by themselves a warning sign. The cleavage geometry is set out on why did my fluorite break.
- Weigh it in water. On a 0.01 g kitchen scale, a clean 20.00 g piece of fluorite weighs 13.70 to 13.72 g suspended in water. The method is on how to measure specific gravity at home.
- Look through a 10x lens. Coatings, dye in cracks, and a glassy skin over a granular body are surface questions, and a lens answers most of them; how to tell if a crystal is dyed covers the checks.
- Scratch test, last. On the base or a hidden spot, a steel point, conventionally about 5.5, marks fluorite at 4. It does not mark quartz at 7. The technique is on how to test mineral hardness properly.
- Optional: an ultraviolet lamp. A glow is consistent with fluorite and its absence proves nothing; the reasons are on is fluorite UV reactive.
The finish state: a piece that passes steps 1, 2 and 4 is consistent with fluorite. A piece that fails any one of them is not fluorite, or is not only fluorite.
Is my fluorite real? The three-number filter, counted over 176 sheets
Of the 176 Handbook sheets in this site's reference set, fluorite is the only one that combines hardness 4, a measured density between 3.0 and 3.25, and a cubic crystal system. Method: the 176 sheets were converted with pdftotext -layout and the Hardness and D(meas.) lines read by script, including the older sheets' printed ranges. Eight sheets print no numeric hardness (n.d. or Soft), which leaves 168.
- 28 of 168 have a hardness range that includes 4, among them aragonite, azurite, malachite, rhodochrosite, smithsonite and sphalerite.
- 2 of those 28 print a measured density that overlaps 3.0 to 3.25: fluorite at 3.175 to 3.184 and magnesite at 3.0.
- 1 of those 2 is cubic: fluorite. The magnesite sheet is hexagonal, hardness 3.5 to 4.5.
The density window is the International Gem Society's figure for massive fluorite with impurities, 3.0 to 3.25, which contains the sheet's 3.175 to 3.184. The filter proves nothing about a specimen that is not in the set, and it does not see coatings, which is why step 3 exists.
The 3.0 floor: fluorite against quartz and calcite on a 20 g piece
A 20.00 g fluorite weighs at least 13.33 g suspended in water, and quartz and calcite weigh 12.45 g and 12.62 g. Worked: weight in water is 20.00 minus the volume, and the volume is 20.00 divided by the density. At the sheet's 3.175 to 3.184 the volume is 6.30 to 6.28 cm³ and the water weight 13.70 to 13.72 g. At the 3.0 floor it is 6.67 cm³ and 13.33 g. Quartz at 2.65 gives 7.55 cm³ and 12.45 g; massive quartz, at 2.59 to 2.63, gives 12.28 to 12.40 g. Calcite at 2.7102 gives 7.38 cm³ and 12.62 g. The closest pair, fluorite against calcite, is 1.08 g apart at the sheet density and 0.71 g apart at the 3.0 floor.
This is the check for purple crystals sold as amethyst, and the other way round. The International Gem Society says fluorite has been offered as amethyst, emerald and colour-change garnet. Amethyst is a variety of quartz, so the scale separates the two without a mark on the piece. Matrix spoils it: a crystal on rock weighs as the rock. A second opinion on colourless material is how to tell calcite from quartz.
Real but treated: coatings, dye and plastic
A fluorite can be genuine and still not be as it came from the ground. The International Gem Society lists heating and irradiation for faceted fluorite, and coating, dyeing and plastic impregnation for decorative objects. A GIA lab note describes a large fluorite sphere under a colourless plastic coating that held a phosphorescent powder. The coating showed itself by the gas bubbles trapped in the plastic; beneath it, cleavage cracks in unrelated directions pointed to a crystalline aggregate, and Raman spectroscopy identified the substrate as fluorite.
So the question has three answers, not two: natural and untreated; fluorite but treated, which is a disclosure question; or not fluorite at all, which the three numbers catch. The wider version of that split is on how to tell if a crystal is fake. Species data for fluorite are on the fluorite species page and the rest of the species index. Nothing here is offered for sale; the wanted list is the only commercial route.