Understanding gems
Treatments explained
Almost every coloured gemstone on the world market has been treated in some way. Treatment is not a dirty word — some methods are centuries old, permanent, and entirely accepted by the trade.
What matters is that you know. The same sapphire, heated or unheated, can differ in price several times over — so an undisclosed treatment is not a technicality, it is the difference between a fair price and a bad one.
Every treatment is disclosed on Gems Guild. Sellers agree to it before they can list, and every gem page states its treatment. If a listing’s treatment looks wrong to you, tell us — we would rather hear it than not.
Untested
Not establishedNobody has established what, if anything, was done to this stone. It is a statement about the evidence, not about the gem.
What it does
Testing for heat means sending the stone to a laboratory, where a gemmologist examines its inclusions under magnification and, where needed, runs spectroscopic analysis. Most stones in the Sri Lankan trade change hands several times without ever going through that, simply because the fee is not worth it against the value of the stone. An untested stone may well be unheated. It may equally be heated. The honest position is that it has not been checked.
What it does to the price
Expect to pay less than for a stone with a laboratory report saying unheated, and that gap is the point: the premium belongs to the evidence, not to the assertion. If the stone matters to you, a report from a recognised laboratory is usually inexpensive next to the difference it settles.
Unheated
No treatmentThe stone came out of the ground with the colour it has now. Cut and polished, nothing more.
What it does
Nothing was done to it. In corundum this is genuinely uncommon: most rough is pale, cloudy or greyish, and only a small fraction is fine enough to sell as it came.
What it does to the price
The premium is large — for fine Ceylon sapphire an unheated stone commonly sells for several times the price of the same stone heated. You are paying for rarity, not for a better-looking gem.
Heat treatment
Long acceptedThe stone was heated to high temperature to improve its colour and clarity. The oldest and most common treatment in the trade.
What it does
Heating to roughly 800–1,800°C rearranges the stone internally: it can deepen or lighten colour, clear the milky 'silk' of fine rutile needles, and even out colour zoning. Sri Lankan cutters have heated stones for centuries — the practice long predates any modern laboratory.
What it does to the price
A heated stone is worth less than an identical unheated one, often substantially. But heating is not damage: the change is permanent and stable, the stone will not fade or revert, and most fine sapphires on the world market are heated. It is normal, and priced accordingly.
Oiling
Expected for the speciesColourless oil or resin fills surface-reaching fissures, making them less visible.
What it does
Emerald almost always reaches the surface with fine fissures. Filling them with oil brings their refractive index closer to the stone's, so they catch the light less and the stone reads cleaner. The trade grades the amount as none, minor, moderate or significant.
What it does to the price
Minor oil is expected in emerald and priced as normal. Heavier filling reduces value, and an emerald with no filling at all commands a real premium.
Living with it
Oil is not permanent. Never put an oiled stone in an ultrasonic cleaner or steamer, avoid solvents and heat, and expect that it may need re-oiling in time. Warm water and a soft brush only.
Irradiation
Materially affects valueRadiation alters the stone's colour centres, changing its colour. Usually followed by gentle heating.
What it does
Most commonly seen in topaz — the strong blues on the market are nearly all irradiated, since natural stones that colour are rare. Also used on some quartz and diamond.
What it does to the price
An irradiated stone is worth a fraction of a naturally coloured equivalent. Because the material is common and the treatment routine, prices are modest and stable — the important thing is simply knowing which you are buying.
Living with it
Treated stones sold legitimately are tested and safe to wear. Some colours can fade with prolonged strong sunlight, so store them out of direct light.
You cannot tell by looking — and neither can we
Heating leaves traces measured in microns: melted haloes around inclusions, dissolved silk, discoid fractures. Seeing them takes a microscope and a trained eye; confirming them takes spectroscopy. Nobody can determine treatment from a photograph, and no honest dealer will claim otherwise.
That is exactly why laboratory reports exist, and why an unheated claim on a valuable stone should be backed by one. Sellers here state treatment in good faith under our terms — but for a significant purchase, ask for the certificate and read it yourself.
Treatments you may meet elsewhere
These are not offered on Gems Guild — sellers cannot select them. They are here because they are usually the answer when a stone somewhere else looks impossibly cheap.
Lattice (beryllium) diffusion
Colour driven into the stone from outside at high temperature, rather than drawn out of it. It can turn dull corundum a vivid orange or yellow. The colour is often only skin-deep, and the stone is worth a small fraction of a naturally coloured one. Detectable only by advanced laboratory analysis.
Lead-glass filling
Heavily fractured ruby impregnated with molten lead glass, turning near-worthless material into something that looks like a solid gem. Extremely fragile: household chemicals, a jeweller's torch and even lemon juice can damage it. Worth a tiny fraction of untreated ruby, and it is what most suspiciously cheap 'rubies' actually are.
Dyeing
Colour added to porous or fractured material. Common in low-grade jade, turquoise and some quartz. Rarely permanent, and it can bleed or fade.