How to tell if a gemstone is real: natural, synthetic and simulant explained

"Real" is the wrong question, and it is why so many people get confused. A lab grown ruby is chemically, physically and optically a real ruby. A red glass paste is not a ruby at all. The useful question is which of three things you are holding: natural, synthetic, or simulant.

The three categories

Natural. Formed in the ground. Comes with the internal fingerprints of a slow, messy geological process.

Synthetic, also called lab grown or created. The same mineral with the same chemistry, crystal structure and hardness, made in a factory. A synthetic sapphire is corundum, exactly like a natural sapphire. It is not fake. It is worth a small fraction of the natural equivalent, because supply is unlimited.

Simulant. Something that merely looks like the stone. Glass, cubic zirconia, synthetic spinel, doublets. Different chemistry, different hardness, different behaviour.

The word to watch in a listing is created. "Created sapphire" and "lab created ruby" are honest descriptions of synthetics. "Sapphire" with no qualifier should mean natural. If a listing says "genuine created" you are looking at a synthetic.

Synthetics are older than most people think

This surprises people who assume anything antique must contain natural stones. Flame fusion synthetic corundum has been commercially available since around 1900, and it was used heavily in ordinary jewelry from the 1920s onward. Synthetic ruby and sapphire in a 1930s ring is normal, period correct and not a sign of a reproduction.

Synthetic emerald arrived in the 1930s and 1940s, synthetic spinel was used constantly as a simulant for many stones, and synthetic star sapphires were popular in mid century American rings.

So a synthetic stone does not date a piece and does not condemn it. It does change the price considerably.

What to look for through a loupe

A ten times loupe and a strong light will answer most questions. You are looking at the inside of the stone, not the surface.

Bubbles. Round or oval gas bubbles almost always mean glass or flame fusion synthetic. Natural stones contain crystals, feathers and fingerprint patterns, but rarely neat spheres.

Curved growth lines. Flame fusion synthetic corundum grows in curved layers, like the lines in a soap bubble. Natural corundum grows in straight, angular zones following the crystal structure. Curved striae are close to proof of a flame fusion synthetic.

Silk. Fine needles of rutile crossing at sixty degrees inside a sapphire are a natural feature and a good sign.

Three phase inclusions. In emerald, a cavity holding liquid, a gas bubble and a tiny salt crystal together is characteristic of Colombian material.

Too clean. A large, completely flawless coloured stone at a low price is more likely to be synthetic or glass than a bargain. Natural emerald in particular is almost always included, to the point that the trade calls the inclusions a garden rather than flaws.

Wear. Look at the facet edges. Glass is soft and its edges go rounded and scuffed. Corundum, at hardness nine, keeps crisp edges for a century.

Doublets and triplets

An older trick that still catches people. Two or three pieces glued together to imitate a single stone.

• Garnet topped doublets were made in quantity in the nineteenth century: a thin slice of red garnet fused to coloured glass. The garnet gives hardness and a convincing surface lustre, the glass gives the colour, so the same top could make a blue, green or purple "stone".
• Opal doublets and triplets back a thin slice of opal with dark material, and sometimes cap it with quartz. Perfectly legitimate when described, much cheaper than solid opal.

To spot one, look at the stone from the side, out of the setting if possible, or through the side of the setting with a light behind. A doublet shows a flat join line and often a slight colour difference above and below it. Immersing the stone in water makes the seam easier to see.

Treatments, which are not the same as fakes

Almost all coloured stones on the market are treated, and most treatment is completely accepted. The question is which treatment, and whether it was disclosed.

• Heat treatment of ruby and sapphire is ancient, permanent, stable and near universal. It improves colour and clarity. An unheated stone with good colour carries a significant premium, and needs a lab report to prove it.
• Oiling and resin filling of emerald is standard practice and has been for centuries. Emeralds are full of surface reaching fractures and filling them makes the stone look clearer. It is expected, but heavy filling should be disclosed and reduces value. It also means never putting an emerald in an ultrasonic cleaner.
• Diffusion treatment drives colour into the surface layer of corundum. Lattice diffusion with beryllium can alter colour throughout. Both are worth much less than untreated or simply heated material and must be disclosed.
• Glass filled ruby is heavily fractured low grade corundum with lead glass poured into the cracks. It looks like ruby and sells cheap. It can be damaged by ordinary jewellery repair heat and by household acids. It is not a stone to pay ruby prices for.
• Dyeing is common in jade, turquoise, lapis and low grade emerald.

What actually drives the price

For coloured stones the order is not the same as for diamonds.

1. Colour. By a long way the biggest factor. Vivid, evenly distributed, neither too dark nor washed out.
2. Clarity, judged by the standard for that species rather than an absolute scale. An emerald is graded against other emeralds.
3. Cut, meaning how well the stone is proportioned to show its colour, not just symmetry.
4. Carat weight, which multiplies everything above rather than standing alone.
5. Origin and treatment, both of which need a laboratory to establish. Kashmir sapphire, Burmese ruby and Colombian emerald carry origin premiums, and untreated material carries a treatment premium.

A one carat stone of superb colour beats a three carat stone of mediocre colour in almost every case.

When to pay for a laboratory report

Get a report when the money justifies it, and let the seller's price guide you.

For anything running into serious money, especially where the value rests on a claim of natural origin, no heat treatment, or a specific source, a report from a recognised laboratory is not optional. The names to look for are GIA, SSEF, Gubelin and AGL. A report from an unfamiliar laboratory with a very confident conclusion is worth less than no report at all.

For an inexpensive piece, a report can cost more than the stone. Judge it with a loupe, buy from somewhere with buyer protection, and accept the small risk.

Home tests worth skipping

Do not scratch test. Hardness testing means deliberately damaging a stone, it is easy to misread, and on anything old it destroys value.

Do not lean on the fog test, the breathe on it and see how fast it clears trick, outside diamond. On a diamond it is a rough screen, since diamond sheds heat fast and the fog lifts almost at once, though moissanite passes it too. On coloured stones it tells you nothing at all.

Be careful with the "too heavy or too light" instinct. Specific gravity is a genuine diagnostic, but it needs the stone out of its setting and proper equipment.

Buying coloured stones second hand

Ask direct questions, and ask them before you buy.

• Is the stone natural or created? Ask in those words.
• What treatments does it have, and how do you know?
• Is there a laboratory report, and from whom?
• Can you photograph the stone under a loupe, and from the side?

A seller who answers "I do not know, it came from my grandmother, here are clear photographs" is being honest and is easy to deal with. A seller who is certain about origin and treatment with no report and no explanation is the one to be careful with.

Then buy where the money is protected. On Nextmine the buyer's payment is held by Stripe until the piece has arrived and been confirmed, so a stone that turns out not to match its description is a recoverable situation rather than a lost bank transfer. Read the mechanics on our how payments work page, or see what is listed right now.

If the stones in question are old cut diamonds rather than coloured stones, our guide to old mine and old European cuts covers what to look for there, and the era by era dating guide explains which stones belong to which period.