Purple quartz, hardness 7, coloured in visible zones. The zoning is what separates it from dyed stone and glass.
Brazil, Uruguay, Zambia, Madagascar, Arizona6 field tests
Amethyst is quartz coloured purple by traces of iron and natural irradiation, and it is the one variety of quartz with a real market. That market is what brings the imitations: dyed quartzite, dyed glass, purple fluorite sold as amethyst, and heat-treated material sold as something it is not.
The good news is that amethyst is one of the easiest valuable stones to verify at home, because its colour has a structure. Iron sits in the crystal lattice unevenly, concentrated in the layers that grew last, so natural amethyst is nearly always zoned — deeper towards the crystal tip, paler towards the base, often in visible angular bands or triangular patches. Dye does not do that. Glass does not do that.
The other thing worth knowing is that almost all commercial citrine is amethyst that has been heated until the iron changed state and the purple went yellow. The two stones are the same material, and a great deal of 'amethyst' geode and 'ametrine' on the market has been through an oven. That is not fraud when disclosed, and it is very common when it is not.
Amethyst is quartz, so every quartz property applies unchanged — the colour is the only thing that differs, and the colour is the least reliable property there is. Which means the identification runs in two stages: prove it is quartz, then prove the purple is natural.
Stage one takes a knife and a glass plate. Stage two takes a loupe and good light, looking for zoning and for dye in fractures.
| Property | Amethyst | What it separates it from |
|---|---|---|
| Mohs hardness | 7 | Purple fluorite (4) — much softer, and it cleaves |
| Specific gravity | 2.65 | Purple glass is usually lighter or heavier, rarely exact |
| Cleavage | None | Fluorite cleaves perfectly in four directions |
| Fracture | Conchoidal | Anything that breaks on flat planes |
| Colour distribution | Zoned — banded, angular, stronger at the tip | Dye (in cracks) and glass (perfectly even) |
| Crystal habit | Six-sided prism, pointed termination, often in a druse | Cubic or octahedral fluorite |
| Fluorescence under UV | None to negligible | Fluorite glows strongly — that is where the word comes from |
| Inclusions | Angular veils, healed fractures, no spheres | Glass: round bubbles |
Purple fluorite is the lookalike that catches most people, and it is separated in one second by a steel knife: fluorite is hardness 4 and marks easily.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Hold the crystal up to a bright light and look through it from several angles. Natural amethyst is almost always unevenly coloured — bands, angular patches, chevrons, or simply a deeper tip and a paler base. Dyed quartzite is flatly and evenly coloured or shows dye trapped along grain boundaries. Glass is uniform throughout. Even, wall-to-wall purple in a cheap large piece is the single strongest warning sign there is.
Purple fluorite is the commonest genuine mineral mistaken for amethyst, and it is sold as amethyst often enough that it is worth two seconds of your time. Fluorite is hardness 4 and a steel blade cuts it easily; amethyst at 7 is untouched. If the blade bites, you have fluorite — which also cleaves into flat-faced octahedral fragments, another instant tell.
Dye needs somewhere to go, and it goes into cracks. Under 10× magnification a dyed stone shows lines and webs of concentrated colour that follow the fracture pattern, with paler material between them. Natural amethyst's colour is indifferent to where the stone is broken.
A perfect sphere of gas inside a stone can only have come from a melt, so a round bubble means glass, every time. Natural amethyst carries angular two-phase inclusions, feathery healed fractures and internal veils, but not spheres. Cheap 'amethyst' spheres and eggs with a few round bubbles inside are moulded glass.
Quartz conducts heat away from your skin better than glass does, so a genuine amethyst feels cool for noticeably longer when you first pick it up. It is a soft test and easy to fool yourself with, but it is free and it is a useful cross-check when the loupe was inconclusive.
Not a test to run deliberately, but a fact worth knowing: amethyst's colour fades in prolonged direct sunlight, permanently. Dyed material fades faster and more unevenly. Either way, a windowsill is the wrong place for a purple stone, and a suspiciously pale patch on one side of an otherwise deep specimen tells you where it has been sitting.
The money in amethyst is small enough that nobody synthesises it for gain in volume — synthetic amethyst does exist and is essentially undetectable without a laboratory, but it is also cheap, so the economic incentive is thin. What is common at the retail end is much cruder: dyed pale quartz and quartzite, moulded purple glass, purple fluorite sold under the wrong name, and heat-treated material misrepresented.
The pattern to hold in mind is that every one of these frauds fails on either hardness or colour structure. Dyed quartzite has the right hardness but the wrong colour distribution. Fluorite has plausible colour but the wrong hardness. Glass fails both, plus bubbles. Run the knife and the loupe and you have covered the field.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
Specimen 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 | What moves it |
|---|---|---|
| Tumbled amethyst, per piece | $2 – 8 | Sold by weight |
| Small geode or cluster, hand-sized | $15 – 80 | Colour depth and crystal condition |
| Large cathedral, deep Uruguayan | $400 – 3,000+ | Height, colour, unbroken points |
| Faceted amethyst, clean, 1 – 5 ct | $5 – 30 a carat | Saturation; 'Siberian' deep purple tops it |
| Purple fluorite sold as amethyst | Should be a fraction of the above | It is a different, softer mineral |
Amethyst is common enough that saturation does nearly all the pricing. Depth of colour, evenness across a specimen and unbroken crystal tips are what a buyer is paying for.
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.