Generally there are three major methods to artificially alter the color of a diamond: irradiation with high-energy subatomic particles; the application of thin films or coatings; and the combined application of high temperature and high pressure (HTHP). However, there is recent evidence that fracture filling is not only used to improve clarity, but that it can be used for the sole purpose to change the color into a more desirable color as well.
The first two methods can only modify color, usually to turn an off-color Cape series stone (see Material properties of diamond: Composition and color) into a more desirable fancy-colored stone. Because some irradiation methods produce only a thin "skin" of color, they are applied to diamonds that are already cut and polished. Conversely, HTHP is used to modify and remove color from either rough or cut diamonds—but only certain diamonds are treatable in this manner. Irradiation and HTHP treatments are usually permanent insofar as they will not be reversed under normal conditions of jewelry use, whereas thin films are impermanent.
Irradiation
Sir William Crookes, a gem connoisseur as well as a chemist and physicist, was the first to discover radiation's effects on diamond color when in 1904 he conducted a series of experiments using radium salts. Diamonds enveloped in radium salt slowly turned a dark green; this color was found to be localized in blotchy patches, and it did not penetrate past the surface of the stone....
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By the way, since you work for a diamond manufacturer, I'd love to get your take on @djevoke's comment about conflict-free diamonds in the comment below!
Dear Devora and djevoke,
Disclaimer: I work for Peled Diamonds LTD. (PeledDiamonds.com)
On May 2000, the Kimberly process was launched.
Most of the countries in the world enforce it and check the origins of every rough diamonds shipments they let into their borders.
As a dealer, you have to sign for every diamond which you import that it’s not a conflict diamond and bear the responsibility for it.
You can’t avoid the process and it’s strict requirements.
By the way, the Israeli Diamonds Exchange was the first in the world to completely boycott the blood diamonds.
As far as I know, since the Kimberly process was launched, the number of blood diamonds that manage to infiltrate the industry was decreased tremendously.
They say that before the process it was about 4% of all diamonds.
Maybe I’m naïve, but I’d like to believe that like you, no one wants to be a part of the injustice and horrors caused by conflict diamonds.