STORIES - Gemstone Treatments: Heated, Oiled, Coated

 

Almost every coloured gemstone you will ever be shown has been touched by human hands before it reached you; not cut and polished, but treated, in ways designed to bring out a colour or clarity that nature did not quite finish on its own. This is not a secret, and it is not automatically a flaw. But it does affect value, and it is exactly the kind of detail worth understanding before you buy.

Why Treatments Exist at All

Gemstones form deep within the earth, under conditions no two stones ever share exactly. The result is enormous natural variation, a ruby might emerge slightly pale, an emerald slightly clouded, a sapphire uneven in tone. Treatments are the industry's long-standing way of consistently improving what nature produced inconsistently, making stones more vivid, clearer, or more durable before they are set into jewellery.

The key distinction is not whether a stone has been treated, but which treatment was used, and how permanent and disclosed that treatment is.

Heat Treatment

Heating is the oldest and most widely accepted treatment in the gemstone world, used for centuries on stones like sapphires and rubies. Under controlled temperatures, heat can dissolve internal impurities, intensify colour, and improve clarity; turning a murky sapphire into a rich, saturated blue.

Because the effect is permanent and the process does not add any foreign material to the stone, heat treatment is considered standard practice and has only a modest impact on value compared to an equivalent untreated stone. The rarer and more valuable case is a stone with excellent colour and clarity that required no heat at all, those command a real premium precisely because they occur so infrequently.

Oiling

Oiling is most closely associated with emeralds, which naturally form with small internal fractures reaching the surface. A colourless oil (traditionally cedar oil, though synthetic resins are also used) is worked into these fractures, making them far less visible and giving the stone a cleaner, more transparent appearance.

Unlike heat treatment, oiling is not permanent. The oil can dry out or seep out over time, particularly with exposure to heat or harsh cleaning, meaning the fractures can become visible again. This is why emeralds should never be cleaned with an ultrasonic cleaner or hot water, and why oiling has a more noticeable effect on value: a minimally oiled emerald is worth considerably more than one that has been heavily treated to disguise significant fractures.

Coating

Coating is the most visible intervention of the three. A thin layer (sometimes containing traces of another material, such as cobalt for blue topaz-style colours) is applied to the surface of the stone to alter or intensify its colour. Because this layer sits on top of the stone rather than working within it, it is also the least durable treatment: coatings can scratch, wear, or chip away with everyday use.

Coated stones sit at the more affordable end of the market, and for good reason. The colour is not intrinsic to the stone itself, which is precisely why disclosure matters so much here, a coated stone should never be priced or represented as though its colour occurred naturally.

What This Means When You're Buying

None of these treatments make a stone "fake", they are industry-standard practices, and the vast majority of coloured gemstones on the market have undergone at least one. What matters is transparency. A reputable jeweller will always tell you whether, and how, a stone has been treated, because that information directly affects both the price you should expect to pay and the care the piece will need going forward.

The rule of thumb is simple: the more a treatment does to the stone, and the less permanent it is, the more it should be reflected in the price. An untreated or lightly heat-treated stone will always be rarer (and more valuable) than one that has needed oil or a coating to achieve its final colour.

Buying with Confidence

Treatments are part of the story of almost every coloured gemstone, not a reason to be wary of one. The real safeguard is simply asking the question and choosing a jeweller who answers it honestly, every time.