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Amethyst vs Fluorite: How to Tell the Two Purple Stones Apart

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: amethyst is purple quartz (silicon dioxide, Mohs 7); fluorite is calcium fluoride (Mohs 4). Amethyst scratches glass and breaks with curved, shell-like fractures; fluorite cannot scratch glass, splits into flat triangular faces, and is about a fifth heavier for its size. Raw crystals are easy: amethyst grows six-sided points, fluorite grows cubes and octahedra. Polished pieces need the hardness, weight or UV checks below.

Why do people confuse amethyst and fluorite?

In a tray of tumbled stones or a row of polished spheres, purple fluorite and amethyst can look like siblings. Both are transparent to translucent, both take a glassy polish, and both commonly show uneven purple color with lighter and darker zones. Minerals.net says outright that fluorite can have the same color as amethyst, and that few other gems match it. Shops that buy mixed parcels sometimes label by color alone, so a "purple stone" bin is where most mix-ups begin.

The underlying minerals could hardly be more different. One is a silicate in the quartz family, the other a halide in the same chemical class as rock salt. That difference shows up in every physical test, which is good news for a buyer.

Amethyst and fluorite side by side

Property Amethyst Fluorite
Mineral Quartz, purple variety Fluorite (fluorspar)
Formula SiO2 CaF2
Crystal system Trigonal (often listed as hexagonal) Isometric (cubic)
Typical crystal Six-sided prism with a pointed termination Cube, octahedron, sometimes dodecahedron
Mohs hardness 7 4
Specific gravity 2.65 3.0 to 3.25
Cleavage None; conchoidal fracture Perfect in four directions (octahedral)
Refractive index 1.544 to 1.553, doubly refractive About 1.433, singly refractive
UV response Usually weak Often blue to violet, many inert
Typical color Lilac to deep reddish purple Purple, green, blue, yellow, colorless, often several in one piece

Data from Webmineral, fluorite, Webmineral, quartz and the International Gem Society amethyst guide, which gives amethyst a refractive index of 1.544 to 1.553 and a specific gravity of 2.651.

What tests separate amethyst from fluorite?

Start with the gentlest test and stop when you get a clear answer. Tests 1, 3 and 4 do no harm at all.

1. The shape test (raw crystals only)

Look at any natural crystal face. GIA describes the classic amethyst crystal as a six-sided columnar crystal with a pyramid-shaped termination. In a geode or cluster you usually see only the pointed tips, but they are always six-sided. Fluorite grows in square-faced cubes, eight-faced octahedra, or blends of the two, and Minerals.net notes it often forms penetration twins where one cube grows through another. Square outlines mean fluorite. Six-sided points mean quartz.

2. The scratch test (use on a hidden spot)

Amethyst at Mohs 7 scratches ordinary window glass. Fluorite, the reference mineral for 4 on the Mohs scale, will not, and a steel knife or nail will mark it. Minerals.net's identification notes list quartz as much harder (7), no cleavage. Press a sharp edge of the stone against a glass bottle and drag gently. A clean bite with a gritty sound points to amethyst. Do this only on raw material or an unseen spot, never on a polished sphere's face.

3. The break test (read the damage)

Every stone has some chip or bruise. On amethyst, chips are rounded and shell-like, with a glassy, rippled surface. On fluorite, chips are flat and stepped, because the crystal splits cleanly along four directions. Geology.com calls fluorite the only common mineral with four directions of perfect cleavage, often breaking into pieces with the shape of an octahedron. Under a loupe, internal fluorite cracks also show as flat, mirror-like planes that flash at matching angles; amethyst cracks are curved.

4. The weight test

Fluorite is noticeably dense. Webmineral averages it at 3.13, against about 2.62 to 2.65 for quartz. Two spheres of the same diameter, one of each, differ by roughly 18 to 20 percent in weight. If you own a kitchen scale that reads to 0.1 g, a simple water-displacement specific gravity check settles almost any polished piece: anything near 3.1 is fluorite; anything near 2.65 is quartz.

5. The UV test

Under a UV flashlight, many fluorites glow blue to violet. Geology.com notes that fluorite typically glows a blue-violet color under short-wave and long-wave ultraviolet light, and that many specimens do not fluoresce at all. A strong blue glow points to fluorite. No glow proves nothing. See the fluorescence guide for which lamp to use.

6. The color-pattern test

Amethyst zoning follows the crystal's geometry. GIA says amethyst commonly shows color zoning in angular zones of darker to lighter color, and in geode pieces the tips are usually the deepest purple. Fluorite banding is often more dramatic: straight bands of purple, green, blue and colorless layered parallel to cube or octahedron faces. A single stone that switches from purple to green is almost always fluorite, sold as rainbow fluorite.

For gemologists

On a refractometer, fluorite reads a single, unusually low index near 1.433 and stays dark between crossed polarizers, because it is isotropic. Amethyst reads two indices around 1.544 and 1.553 and blinks light and dark as it rotates in a polariscope.

How does each one form, and where does it come from?

Most commercial amethyst grows in gas cavities in basalt lava flows. Geology.com describes how, in Brazil and Uruguay, large cavities can contain hundreds of pounds to several tons of amethyst crystals. That is where the big cathedral geodes and clusters come from. Minerals.net names Brazil as the largest producer, with other important deposits in Uruguay, Bolivia, Zambia, Namibia and elsewhere. The purple itself comes from iron in the crystal plus natural radiation; the why amethyst is purple guide explains the chemistry.

Fluorite grows from hot, fluorine-bearing water. Geology.com notes that most fluorite occurs as vein fillings in rocks altered by hydrothermal activity, often alongside ores of lead, zinc, copper, silver and tin, and also in fractures and cavities in limestone and dolomite. Classic localities are mining districts: Weardale in England, Hardin County in Illinois, the Elmwood mine in Tennessee, and many Chinese mines. Location alone can settle a label. A purple stone from a Uruguayan basalt geode is amethyst; a purple cubic cluster from a lead-zinc mine is fluorite.

Which one costs more?

Neither is rare in ordinary grades, and that shapes pricing. GIA notes that relatively plentiful supplies have made amethyst widely available, and that its price per carat does not rise dramatically with larger size. Clean, deep, evenly colored amethyst costs more than pale or zoned material, and brown tints lower value.

Fluorite prices swing more by specimen than by stone. Tumbled and massive fluorite is among the cheapest colorful material on the market. Crystal specimens are another story: Minerals.net observes that classic fluorite occurrences can command exorbitant prices. A sharp purple cube cluster from a closed American mine can outprice an amethyst geode many times its size.

Are either treated or faked?

Both have their tricks, and they differ.

  • Amethyst can be heated. The IGS reports that amethyst heated to 400 to 500 °C may turn brown, red and sometimes green, and that heated yellow material is sold as citrine. GIA adds that light-colored quartz has been dyed purple, and synthetic amethyst is grown hydrothermally. The amethyst authenticity guide covers those checks.
  • Fluorite is mostly sold untreated, but Minerals.net notes that some dealers oil fluorite specimens to enhance luster, and that some fluorite color caused by hydrocarbons can be removed by heating. Fluorite's bigger risk is misidentification, in both directions.

The surprise: a fluorite sold as amethyst is not a fake in the usual sense. It is a natural stone with the wrong name, which still matters, because it is far softer and needs different care.

Which should you buy, and how do you care for each?

Buy amethyst for anything that will be handled or worn. At hardness 7 it survives a pocket, a ring or a desk. Buy fluorite for display: color range, crystal geometry and UV glow make it one of the best cabinet minerals. If you want a purple stone you will carry daily, choose amethyst; a fluorite palm stone will collect scratches and chips within months.

Care follows from hardness. Keep fluorite away from harder stones, avoid ultrasonic cleaners and sudden temperature changes (both can open its cleavage planes), and dust it rather than scrubbing. Amethyst tolerates soap and water but should stay out of long direct sun; Minerals.net notes some amethyst may slightly fade with prolonged exposure to light.

In crystal traditions, amethyst's name comes from the Greek for "not drunken," while fluorite is associated with order and focus. Fluorite's own name is practical: the Natural History Museum of Utah explains it was named in 1797 from the Latin "fluere," to flow, because it was added as a flux in glassmaking and iron processing.

Shopping amethyst and fluorite we stock

When buying either from photos, look for a listing that shows natural crystal faces or at least one chip, since those settle identity at a glance. For polished pieces, a stated weight and diameter lets you estimate specific gravity before you buy.

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Amethyst and fluorite in stock

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Frequently asked questions

Is purple fluorite the same as amethyst?

No. Purple fluorite is calcium fluoride, a soft halide mineral at Mohs 4. Amethyst is quartz, a silicate at Mohs 7. They can share a color, but they differ in crystal shape, hardness, density, cleavage and optics, and they form in different geological settings.

How can I tell amethyst from fluorite without tools?

Look for a natural face or chip. Six-sided points and curved, shell-like chips mean amethyst. Cubes, octahedra, and flat stepped chips mean fluorite. A purple stone that also has green or blue bands is almost always fluorite. If you can, compare the weight of similar-sized pieces: fluorite feels heavier.

Does amethyst glow under UV light?

Amethyst usually shows little or no response under a UV flashlight. Many fluorites glow blue or violet, which makes a UV lamp a quick hint. It is not proof, because plenty of fluorite is inert. Combine the UV check with hardness or weight before deciding.

Which is more fragile, amethyst or fluorite?

Fluorite, by a wide margin. It is three steps softer on the Mohs scale and splits along four cleavage directions, so knocks chip corners and edges. Amethyst is hard and has no cleavage, though a sharp blow can still fracture it. Fluorite belongs on a shelf; amethyst can be carried.

Can fluorite be sold as amethyst?

It happens, usually through careless sorting of tumbled stones or beads rather than deliberate fraud. A seller who mixes them up likely has not tested the stock. Ask for the hardness or weight of a polished piece, or choose listings that show crystal faces.

Sources