The primary role of laboratories is to certify whether colorless diamonds are natural, and to grade them. In order to do this, four primary criteria (the 4 Cs) are taken into account: Carat (weight), Color, Clarity, and Cut. All of this information is laid out in a document (a "certificate") that is joined to the stone and will be used to evaluate the price of a diamond. So the certificate does not in itself determine the price of a stone, but it does certify its essential attributes and most of all, it constitutes a guarantee that the stone is of natural origin.
In the past 20 years, the laboratories have expanded their activities to include precious stones and natural color diamonds. They brought their experience in colorless diamonds to bear on natural color diamonds, without developing criteria and policies that are really adapted to the latter - with sometimes catastrophic results, in particular in relation to the definition of color. Given their extreme rarity, color diamonds only bring small revenues to the laboratories, who consequently have no interest in investing heavily in research or equipment.
The laboratories only deliver a certificate if the diamond and its color are natural. All synthetic diamonds and diamonds whose color has been artificially altered (except for the HPHT process) are excluded.
If there is the slightest doubt about the natural origin of a color, the word "undeterminable" is put on the certificate. This makes the diamond unsellable and constitutes a real loss for the diamantaire who, having usually purchased the rough stone as it came out of the mine, can be certain of the natural color of the diamond - unless it was treated in the rough. It can happen that diamantaires have already had their stones classified differently by different laboratories - "treated color" by some, and "natural color" by others. One can only hope that in the future, technological progress will allow us to establish the truth with more certainty.
The main laboratories are: GIA (Gemological Institute of America), IGI (International Gemological Institute), HRD (Hoge Raad voor Diamant - Antwerp Diamond High Council), EGL (European Gemological Laboratory), AGS (American Gem Society), CGL (Central Gem Laboratory Japan)…