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Precious stones
Semi-precious stones
Technical information
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  Optical properties
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  Crystal systems
History of the stones

The demand for stones of good quality (beautiful colors, no inclusions) is largely higher than what the production can provide. So the great majority of the stones are treated.
If the treatments by heating or colourless oiling are tolerated (they are centuries old), the other treatments must be imperatively mentioned, which is not always the case!

Heating :
This technique is certainly oldest (3000 years?). It consists in heating the stone at temperatures going of a few hundreds of degrees to more 1800° in order to improve the color and the purity of it. If at the beginning the heating took place on embers on which one blew, one uses electric furnaces now.
The heating makes it possible to carry out all kinds of operations on the color: to reduce, reinforce it, change it, standardize it, eliminate a nuance…

The stones are subjected to various types of radiations (UV, alpha, beta, gamma, x-ray, microwaves….) in order to change or to improve the color.
The stability of the color and the reversability of the treatments depend on the rays used.
The stones which undergo this type of treatment are mainly topazes (blue colouring: “Sky blue”, “Swiss blue”, “London blue”…) and diamonds (colouring: “Color enhanced”).
It should be known that the stones subjected to certain treatments, in particular those containing neutrons, become radioactive and are, so dangerous to wear!


It can be done on the surface or in-depth. The diffusion is a particular heat treatment of the stones.

  • On the surface: the colourless or very pale stone is heated in a crucible until 1600°/1800° with a mixture of dyes making it possible to have a layer of color on the surface.
  • In-depth: the stone is heated with more 1800° in an atmosphere rich in oxygen and another element which will use to colour like beryllium (more used in particular on sapphires) or cobalt.

The cracks and cavities are filled by an oil (coloured or not) having an index of refraction close to that of the stone to be treated.
This treatment is used primarily with the emerald (oil of cedar, balsam of Canada). In the event of degradation of oil, it can be replaced.
Attention, the filling of the cracks is done more and more with synthetic resins (opticon, loctite) with disastrous effects for the stone such of the cracks which increase, of inclusions which make darker, impossibility of removing the resin.

Dyeing and colouring:
The dyeing is a surface treatment which makes it possible to reinforce the color of the stone.
The colouring was a technique usually employed at the XIXth century in Russia which consisted in colouring the backside of the gem (primarily on emeralds) but which fell in disuse.

Other treatments:
New treatments are relatively new:
  • Laser drilling : improvement of the purity of diamonds (“Clarity enhanced”)
  • HPHT process: High Pressure and High Temperature treatment to change the color of brown diamonds into white diamonds (from G to J)