Heated, oiled, filled: treatments and what they cost you

Treated is not the same as fake

Almost every coloured stone sold today has been treated, and that is not a scandal. Treatment is an accepted part of the trade provided it is disclosed, and disclosure is expected on the invoice. A heated sapphire is still natural corundum; a furnace finished a job the earth started. A glass imitation is a different object altogether, and the two questions should never be run together.

What treatment changes is price, by very different amounts depending on what was done. Some treatments are so routine that the market prices them as the default and pays a premium for their absence. Others leave a stone fragile, or dependent on a layer of colour that ordinary wear can remove, and the right price is a small fraction of an untreated equivalent. In nearly every case you cannot see which is which. The treatments that move the money most are invisible to the eye, to a loupe and to a photograph.

Heat, and the premium for an unheated stone

Heating ruby and sapphire is among the oldest practices in the gem trade, and it has been done for as long as there has been a trade. Held at high temperature, corundum can shed a cloudy grey cast, dissolve fine silk inclusions and settle into a stronger, more even colour. The change is permanent and structural. There is no coating to wear off and nothing to dry out, and a heated sapphire cleans, resizes and wears exactly as an unheated one does.

Because heating is so nearly universal, the market assumes it. A fine ruby or sapphire with a laboratory report stating no indications of heating sells at a large premium over a heated stone of the same apparent colour, and on important stones that premium can dominate the price. It is not paid for beauty – the heated stone may look identical – but for rarity, because a stone that reached that colour unaided is uncommon. You cannot judge it by looking: a gemmologist reads the inclusions under magnification for the signs of high temperature, and a laboratory adds spectroscopy.

Oil in emeralds, and the grade of it

Emeralds grow with fissures that reach the surface, and nearly all of them are filled with a colourless substance close to the stone’s own refractive index, so those fissures stop catching the light. Traditionally this is cedarwood oil; modern practice also uses artificial resins. The stone reads clearer and greener. This is expected, it is disclosed as clarity enhancement, and an emerald with none at all is unusual enough to carry a premium of its own.

Two details matter more than the fact of it. The first is degree: laboratories report enhancement as none, minor, moderate or significant, and that scale reflects how much of the apparent clarity is filler rather than stone. The gap in price between minor and significant is wide.

The second is that oil is not permanent. It can dry, shrink or seep out over decades, and fissures that were invisible become visible again. Heat, steam, solvents and ultrasonic cleaners all hurry it along. Nothing structural has been lost, and a jeweller who works with emeralds can clean and re-oil the stone. But it means warm water, mild soap and a soft brush, and nothing else.

Lead-glass filling is a different thing entirely

Filling a ruby with lead glass is not the improvement of a good stone. It is a process applied to heavily fractured, low-grade corundum that would otherwise be unsaleable. The material is heated with a lead-rich glass that flows through the fracture network and sets, bonding the pieces into something that holds together and looks like a red stone. In the worst material the glass is a large part of what you are holding.

Such stones are worth a small fraction of a comparable heated ruby, and the gap is not a matter of taste. Lead glass is soft and chemically vulnerable in ways corundum is not.

• Ordinary bench heat during a repair – sizing a shank, retipping a claw – can cloud, pit or collapse the filling.
• Lemon juice, household descalers and some jewellery dips etch the glass.
• Ultrasonic and steam cleaning can drive the filling out.

Anyone working on the stone has to be told first, which is only possible if you know. Under magnification a gemmologist sees trapped gas bubbles in the filled channels and a blue-orange flash along them.

Diffusion, and colour that lives in a layer

Diffusion heats a stone in contact with a colouring element so that colour develops near the surface rather than through the body of the material. Titanium diffusion produces blue in sapphire; beryllium diffusion pushes corundum towards orange and yellow, sometimes reaching deeper.

The consequence is practical. Because the colour sits in a layer, anything that removes surface removes colour. A chip on the girdle can expose pale material underneath, and a repolish – the usual remedy for an abraded stone – can take the colour off or leave it patchy. That is not a thing to discover at a bench years later.

Irradiated and heated blue topaz belongs in a different category, and it is worth saying plainly because it worries people unnecessarily. Nearly all blue topaz began as near-colourless material, was irradiated and then heated. The colour is stable, the material is ordinary topaz, and where the treatment is regulated stones are held and checked before sale. It is not a treatment to be anxious about. It is simply why blue topaz is inexpensive.

Dye, stabilisation and impregnation

Porous and aggregated materials attract treatments of their own.

• Jade. Bleached and polymer-impregnated jadeite, and dyed jadeite, are both common. The trade separates untreated material from bleached, dyed and both, and the price difference is enormous. Dye fades and polymer can yellow.
• Lapis lazuli. Pale or grey material is dyed a deeper blue and sometimes waxed, and the dye can transfer to a cloth.
• Turquoise. Most turquoise on the market is stabilised with resin, which hardens chalky material and stops it absorbing oils and shifting colour. Stabilised turquoise is a legitimate product; dyed or reconstituted material is a much cheaper one.
• Pearls. Bleaching is routine and dyeing common, and both should be stated.

None of this makes a piece worthless. It makes the description important, which is why the wording on an invoice matters as much as the stone.

What a photograph can do, and what settles it

A photograph carries some of what treatment leaves behind: colour that reads too even for the material, a flash of the wrong colour along a fracture, a chipped girdle showing paler stone beneath, dye pooled in a crack. Those are grounds for a question, never an answer. Carat records observations like these as treatment signs with the reasoning attached, and where the evidence runs out it says so rather than guessing.

What settles it is a gemmological laboratory: magnification, spectroscopy and, for the harder cases, chemical analysis. For a coloured stone of any real value that report is what a serious buyer asks to see, and it is the difference between selling a stone as what it is and selling it as what you hoped. To judge whether the fee is worth paying, what your jewellery is worth sets out the three markets a piece sits in, and how to tell if a diamond is real covers the separate treatments found in diamond.