Guide to gem treatments
The great majority of gems on the market are “enhanced”. Some treatments have been accepted for centuries, others should send you running. In France, every treatment must legally be declared at the point of sale.
Heating
Near universal on sapphires and rubies to intensify colour. Stable, permanent and universally accepted — a certified “unheated” sapphire even commands a premium.
Oiling
Traditional on emeralds: oil fills the fissures and revives transparency. Accepted when declared — but never an ultrasonic bath, which drives the oil out!
Irradiation
Commercial blue topaz is almost always colourless topaz that has been irradiated then heated. A controlled, stable and harmless process.
Stabilisation
Porous turquoise is often impregnated with resin to harden it and fix its colour. Common and accepted when declared.
Diffusion ⚠
A colouring element (beryllium and the like) is diffused into the surface at high temperature: the colour is only a skin. Must be declared — value is far lower.
Lead glass ⚠
Heavily fractured rubies are “rebuilt” with leaded glass: these fragile composites cannot tolerate lemon, heat or household products. Their knock-down price is justified.
Dyeing ⚠
Howlite dyed as “turquoise”, agates in improbably garish colours: a cotton bud with acetone often reveals the trick.
Coating ⚠
A thin iridescent film deposited on the surface (“mystic topaz”, aurora quartz): spectacular, but it wears with wear and scratches.
Almost every gem on the market is treated
Here is a truth the market does not advertise: the great majority of gems sold are “enhanced”. This is not necessarily scandalous — some treatments have been practised for centuries and are perfectly accepted — but they must be legally declared at the point of sale in France, and they radically change a stone's value.
The accepted treatments
Heating is near universal on sapphires and rubies: it intensifies the colour and dissolves certain inclusions. The result is stable and permanent, and the process mimics what nature does underground. It is so widespread that a certified unheated sapphire commands a substantial premium — rarity has changed sides.
Oiling emeralds is a millennia-old tradition: a colourless oil fills surface fissures and revives transparency. Accepted when declared — but it imposes one absolute rule of care: never put an emerald in an ultrasonic bath, as it drives out the oil and brutally reveals the fractures.
Irradiation affects virtually all commercial blue topaz: originally colourless, it is irradiated then heated. The process is controlled, stable and harmless. Stabilisation with resin, meanwhile, applies to turquoise, porous by nature.
The treatments that should alarm you
Diffusion: a colouring element, often beryllium, is diffused into the surface at very high temperature. The colour is only a skin — a simple repolish can remove it. Value collapses.
Lead-glass filling: heavily fractured rubies, all but worthless, are “rebuilt” with leaded glass. These composites are fragile and cannot tolerate lemon juice, heat or household products. Their knock-down price is justified — provided you know what you are buying.
Dyeing: white howlite dyed to pass as “turquoise”, agates in improbable colours. A cotton bud soaked in acetone on a discreet area often reveals the trick. Finally coating: a thin iridescent film deposited on the surface — “mystic topaz”, aurora quartz — spectacular, but it wears off with wear and scratches easily.
Keep exploring
E-book · Gemmology & the gem trade
The Merchants of Light
My name is Lorys. For over ten years I have travelled the markets, the mines and the workshops of the gem world. There I learned to observe stones, to negotiate, to recognise treatments and to understand what a gem is truly worth. The Merchants of Light is a human and practical journey. You will find field knowledge and professional insight that you will not find anywhere online.
- Travel the great gem routes
- Understand the stone trade
- Negotiate with method
- Learn to read a gem
- Recognise treatments and imitations
- Use the tools of the trade
- Buy with far greater safety
- Step into the professionals' network
- Make sense of certificates