Crystals and polished stones are sold everywhere, from museum shops to market stalls, and most sellers are honest. Even so, a lot of what's on sale has been dyed, heated, coated or simply made in a factory, and it isn't always labelled that way. Knowing the most common substitutions makes them easy to spot.
Fake, treated or synthetic?
It helps to separate three different things, because not all of them are a problem.
- Imitations look like a natural stone but are a different material altogether: glass, plastic, resin, or another cheaper mineral.
- Treated stones are genuine natural material that has been changed to improve its colour or durability, by dyeing, heating, coating or filling. Many treatments are standard in the trade and fine if they're disclosed.
- Synthetic (lab-grown) stones have the same chemistry and structure as the natural mineral but were grown in a lab. They're real crystals, just not natural ones, and should be sold as such.
The problem is never the material itself. It's being sold one thing and given another.
The most common fakes and treatments
Howlite and magnesite dyed as turquoise
Howlite is a white, porous mineral with grey or black veins. Dyed blue, it looks a lot like turquoise, and it's one of the most common turquoise imitations. Magnesite is dyed in the same way. Signs to look for:
- A very even, bright blue, sometimes slightly too vivid
- Colour that's deeper in cracks and veins, and paler in the middle of the stone
- Dye coming off on a cotton bud dampened with acetone (nail polish remover), tested on the back or a drill hole. This can damage some finishes, so test discreetly and only on stones you own.
Reconstituted and stabilised turquoise
Natural turquoise is often soft and porous. Much of what's sold is stabilised, impregnated with resin to harden it, which is a common, accepted treatment if disclosed. Reconstituted turquoise is made from turquoise powder bonded with resin into a block, and "block turquoise" can also be pure plastic or dyed material. Very uniform colour, perfectly regular matrix lines, and a low price are warning signs.
Dyed agate
Agate takes dye easily because it's slightly porous. Bright electric blue, hot pink, purple and vivid green agates are almost always dyed. Natural agate tends toward greys, whites, browns, reds, oranges and soft blues. Dyed agate is still genuine agate, but it should be sold as dyed.
Heated amethyst sold as citrine
Natural citrine (yellow quartz) is relatively uncommon. A great deal of the citrine sold today is amethyst that has been heated, which turns its purple to yellow, orange or brownish-orange. This is easiest to spot in clusters and geode pieces:
- Heated amethyst clusters often have deep orange or reddish crystal tips with a white or pale base.
- Natural citrine is usually a paler, more even yellow to golden colour.
Heat-treated citrine is still quartz, so it isn't a fake in the strict sense, but it isn't natural citrine either.
Glass sold as obsidian
Natural obsidian is volcanic glass, and is usually black or dark brown, sometimes with a golden or rainbow sheen, or with white "snowflake" patches. Transparent, brightly coloured "obsidian" in blue, green, red or purple is usually man-made glass or slag glass. If it looks like a beautiful bottle, it probably is one.
Opalite
Opalite is a man-made glass, milky white with a blue glow against dark backgrounds and an orange tint against light ones. It's often sold under its trade name without mention that it's glass, and sometimes passed off as moonstone or opal. It's an attractive material, but it isn't a natural mineral.
Aqua aura and other coated quartz
"Aqua aura" is natural quartz that has been coated with a very thin layer of gold, bonded to the surface in a vacuum chamber, giving an intense blue sheen. Similar coatings make "rainbow", "titanium" and other aura crystals. The quartz underneath is real; the colour isn't natural. Look for colour that sits only on the surface and wears through on edges and points.
Resin and plastic
Cheap "crystals", especially towers, spheres and novelty shapes, are sometimes cast in resin, occasionally with real chips embedded. Resin is light for its size, feels warm and can be scratched easily.
Goldstone and other glass
Goldstone is glass with tiny copper crystals that glitter inside. It's sometimes sold as a natural stone, though it isn't one. Glass is also commonly sold as clear quartz, rose quartz and "cherry quartz". See quartz, calcite or glass? for how to tell clear materials apart.
Simple checks you can do yourself
| Check | What a natural stone usually shows | What suggests a fake |
|---|---|---|
| Bubbles | Inclusions that are irregular, angular or crystal-shaped | Perfectly round bubbles, swirls, flow lines |
| Colour | Natural variation, zoning, not too saturated | Unnaturally even or vivid colour, dye pooled in cracks |
| Temperature | Feels cold to the touch and warms slowly | Resin and plastic feel warm almost immediately |
| Weight | Feels solid for its size | Resin and plastic feel light |
| Hardness | Quartz (7) scratches glass | Glass (about 5.5) won't scratch other glass easily |
| Surface | Wear or uneven polish on natural material | Mould seams, a coating worn thin on edges |
| Price | Fair for size and quality | Far too cheap for a big, perfect, bright stone |
A few notes on these:
- Bubbles are the most reliable visual sign of glass or resin. Look with a magnifier in bright light.
- The temperature test works best for separating stone and glass from plastic and resin. Glass and quartz both feel cool, so it can't separate those two reliably.
- Hardness is useful but destructive. Only test a hidden spot on a stone you own. Our guide to the Mohs hardness scale at home explains how.
- Price isn't proof, but it's a strong signal. Large, clear, deeply coloured natural stones are uncommon, and they're rarely cheap.
Tip: ask the seller directly whether a stone is natural, treated, synthetic or man-made. Reputable sellers will tell you, and a vague or evasive answer is a useful answer in itself.
When to ask a professional
For jewellery, gemstones and anything expensive, home checks aren't enough. A qualified gemmologist uses instruments such as a refractometer, microscope and spectroscope to separate natural stones from synthetics, imitations and treatments. Lab-grown gems in particular can look identical to natural ones and need proper testing. If value matters, see is my rock valuable?
A photo ID app is a reasonable first check too. AI Stone suggests what a stone is and shows its typical colours, luster, transparency and hardness, so you can compare those against the piece in front of you. It can't detect dye, heat treatment or a lab origin from a photo, though, so treat the result as a starting point rather than a certificate.