How to tell if amethyst is real
Amethyst is quartz, hardness 7, a crystal that reads 1.544 and 1.553 on a refractometer. Glass reads once and shows bubbles. Synthetic needs a laboratory.
To tell if amethyst is real, use a lens, then a refractometer. Amethyst is quartz, hardness 7, and gives two readings, 1.544 and 1.553. Glass gives one, and under 10x often shows round bubbles and swirl marks. A scale cannot clear glass. Synthetic amethyst passes every home test; only a laboratory separates it.
In short
- Real amethyst is quartz, and quartz has fixed numbers. The Handbook of Mineralogy sheet gives hardness 7, density 2.65 and two refractive indices, 1.544 and 1.553. The International Gem Society calls those values very constant, and GIA prints the same pair. The candidates for a fake are glass, another purple mineral and synthetic quartz.
- The two-reading rule: a purple stone that gives two refractometer readings, 1.544 and 1.553, is quartz. Of the 176 sheets in this site's reference set, 127 print a refractive index and 14 touch that window. Quartz is the only one that prints both values and the only one that reaches hardness 7. Glass is singly refractive and gives one reading somewhere between 1.470 and 1.700.
- The lens comes first because it costs nothing. The International Gem Society says glass usually shows swirl marks and round bubbles, and moulded glass shows mould marks, rounded facet edges and concave facets. Natural amethyst shows angular colour zones (GIA), and its typical inclusions are negative cavities, thumbprints, rippled fractures and twinning lines (IGS).
- The scale catches fluorite, not glass. A 20.00 g piece weighs 12.45 g in water as quartz and 13.70 to 13.72 g as fluorite. Glass runs from 11.30 to 15.56 g across the gem society's range, so a quartz-like reading proves nothing about glass.
- Synthetic amethyst is where home tests stop. GIA's Christopher Breeding writes that natural and synthetic quartz can be nearly indistinguishable once faceted. The IGS says synthetic rough is sometimes sold as natural. That question goes to a gem laboratory. No flame or lighter test is given here.
| Check, in order | Natural amethyst (quartz) | Glass | Synthetic amethyst | What the check separates |
|---|---|---|---|---|
| 1. 10x lens | Angular colour zones (GIA); negative cavities, thumbprints, rippled fractures, twinning lines (IGS) | Round bubbles, swirl marks; if moulded, rounded facet edges and concave facets (IGS) | Like most faceted quartz, often free of inclusions (GIA) | Glass, often; synthetic, no |
| 2. Weigh in air and in water, 20.00 g | 12.45 g (D 2.65) | 11.30 to 15.56 g (SG 2.30 to 4.50) | Same as quartz | Fluorite (13.70 to 13.72 g); not glass |
| 3. Hardness, rough or hidden spot only | 7 | 5 to 6 | 7 | Glass, by 1 to 2 Mohs points |
| 4. Refractometer | Two readings, 1.544 and 1.553 | One reading, 1.470 to 1.700 | Two readings, as quartz | Glass and other minerals: yes |
| 5. Polariscope | Doubly refractive, uniaxial (+) | Singly refractive; often anomalous double refraction (IGS) | As quartz | Weakly, because of glass's anomalous effect |
| 6. Gem laboratory | Natural | - | Synthetic, by trace-element and other laboratory methods (GIA) | The only check that separates synthetic |
How to tell if amethyst is real: the six checks, in order
Start with a 10x lens and a single lamp; finish, if you need to, at a gem laboratory. The order runs from free and harmless to costly, and each step removes one kind of fake. Amethyst is purple quartz, and the Handbook of Mineralogy sheet for quartz gives the numbers every check uses: hardness 7, measured density 2.65, uniaxial positive, indices 1.544 and 1.553.
- Lens. Look for round bubbles and swirl marks, which the International Gem Society article on glass gemstones lists as glass inclusions not found in natural gems, and for rounded facet edges or concave facets on a moulded piece. GIA describes natural amethyst's colour zoning as angular zones of darker and lighter colour.
- Scale. Weigh it in air and suspended in water. A 20.00 g quartz piece reads 12.45 g, as on why is my amethyst losing color; the method is on how to measure specific gravity at home.
- Hardness, only on rough or a hidden spot. Quartz is 7 and glass 5 to 6. Never test a faceted stone; how to test mineral hardness properly explains why.
- Refractometer. Two readings, 1.544 and 1.553, means quartz. One reading means it is not quartz.
- Polariscope. Quartz is doubly refractive. Glass is singly refractive, but often shows anomalous double refraction, so this step supports step 4 rather than replacing it.
- Laboratory. For natural against synthetic, nothing earlier on the list decides it.
The finish state is one of four labels: quartz, natural or synthetic still open; glass; another purple mineral; or quartz confirmed natural by a laboratory. For a crystal on its matrix with its growth surfaces intact, step 1 is often the whole job, in our judgement. Treated and dyed material is on how to tell if a crystal is dyed. The identification pages are indexed at the collecting guides.
Is my amethyst real or glass? The two-reading rule, counted
The two-reading rule: a purple stone that reads 1.544 and 1.553 on a refractometer is quartz, and so is amethyst, natural or synthetic. GIA prints the same two indices and a birefringence of 0.009; the International Gem Society gives them as very constant. Glass, on the gem society's figures, is singly refractive with a single index anywhere from 1.470 to 1.700.
Method. The 176 sheets in this site's reference set were converted with pdftotext -layout, and every refractive-index value or range in the Optical Class field was extracted by script. 127 sheets print at least one index. 14 have a value or range touching 1.544 to 1.553: autunite, chalcophyllite, halite, lepidolite, marialite, muscovite, phlogopite, polylithionite, portlandite, pyrophyllite, quartz, talc, variscite and vermiculite. Five of the 14 are uniaxial, and quartz is the only one of the 14 that prints both 1.544 and 1.553, and the only one that reaches hardness 7. The other 13 top out between hardness 1 and 6, ten of them at 3 or below.
What the rule cannot do: it says quartz, not natural. It also needs a polished flat face and a gemmological refractometer, so it suits a cut stone or a polished window better than a crystal cluster. A cluster in its geode is, in our judgement, normally settled by the lens, the crystal habit and the matrix. Other look-alikes are covered on how to tell if a crystal is fake.
Why the scale cannot clear glass: a worked example
A density reading that matches quartz does not rule out glass, because glass spans the whole range around it. The International Gem Society gives glass a specific gravity of 2.30 to 4.50, since lead is added to raise refractive index and also raises density. Worked as weight in water equals mass times one minus one over density, a 20.00 g glass piece reads anywhere from 11.30 g to 15.56 g. Quartz, at 2.65, reads 12.45 g, inside that span.
The scale is still worth using, for a different job. Purple fluorite, density 3.175 to 3.184 and hardness 4, reads 13.70 to 13.72 g, so the scale separates the commonest mineral look-alike. That comparison, the 2.9 line, is set out on the amethyst colour page and how to tell if fluorite is real and is not repeated here. Treat this as a documented negative: the cheapest instrument separates the two minerals and fails on the commonest imitation.
The touch test is weak evidence. The gem society notes that glass conducts heat quickly and feels warmer than the gems it imitates. That is an impression, not a measurement, and it is not used to decide anything on this page.
Is my amethyst real or synthetic? Where home tests stop
Synthetic amethyst is real quartz grown in a laboratory, so it passes the lens, the scale, the hardness point and the refractometer. The International Gem Society says amethyst can be grown hydrothermally, or made by gamma-irradiating specially prepared smoky quartz, and that synthetic rough is sometimes sold as natural. Christopher Breeding of the GIA Laboratory writes that natural and synthetic quartz have distinctly different crystal shapes but can be nearly indistinguishable once faceted.
So a natural crystal shape is evidence; a faceted stone is not. GIA's research tested 38 natural and 21 synthetic amethysts, among other quartz, using laser-ablation trace-element analysis. That is laboratory work, and the gem society's advice is the same: ask a professional gemmologist or a gem laboratory.
Heat-treated amethyst is a different question. The gem society says amethyst heated to yellow or red-orange becomes, by definition, citrine, and that 400 to 500 °C can turn it brown, red or green. That is why no flame, lighter or heat test appears on this page: heat is how amethyst loses its colour. Nothing on this site is offered for sale; the wanted list is the only commercial route.