CIBJO (Confédération Internationale de la Bijouterie, Joaillerie et Orfèvrerie - also known as the World Jewelry Confederation) and government agencies like United States Federal Trade Commission (FTC) ask for the disclosure of diamond treatments performed.
Coloring of a diamond is performed by three methods:
The first two are able to just intensify a diamond color (off colored Cape series stones). Some irradiation methods produce a thin skin of color, therefore these methods are applied to already cut and polished diamonds. But HTHP modifies and removes color and can be performed on both cut and rough diamonds. Only certain diamonds are treatable by this method. Irradiation and HTHP methods are almost permanent in effect unlike thin film method. CIBJO demands disclosure of all color enhancements performed on a diamond.
In 1904, Sir William Crookes discovered radiation's effects on a diamond's color by conducting series of experiments using radium salts, and he observed that the diamonds enveloped in these salts slowly turned dark green and the color appeared in patched and did not penetrate deep into the gem and remained on the surface. This was due to the emission of alpha particles by radium, but the radioactivity was also somehow was retained by the diamond, rendering it as un-wearable. The diamond was donated by Crookes to British Museum in 1914. It has neither lost its color nor the radioactivity till today.
More safer methods were then devised for irradiating diamonds. These include proton and deuteron bombardment via cyclotrons, gamma ray bombardment by exposure to cobalt - 60, neutron bombardment by piles of nuclear reactors and electron bombardment by Van de Graaff generators. These irradiation methods are based on moving the carbon atoms from their place in the crystal lattice and producing a color centre. The colors thus produced are shades of green, blue or black. Other colors after further annealing ( defects of irradiation are reduced by annealing and individual carbon atoms' mobility increases and the overall affect depends on the temperature and time of annealing ) of these diamonds give out bright shades in yellow, orange, brown and pink.
Cyclotroned diamonds have greenish or bluish green color only at their surface. They are later annealed to obtain a yellowish or orange color. Their radioactivity remains for just few hours after the treatment due to the directional nature of the treatment and is imparted only to selected zones of the diamond. This treatment is now uncommon. Gamma ray treatment is also becoming unpopular, though it is the safest and cheapest method, it takes several months for the treatment, and a blue-green color is produced in whole of the gem.
The two most common methods for irradiation are neutron and electron bombardments. Neutron Bombardment produces green to black shades penetrating the whole stone. Electron Bombardment produces blue, blue-green or green color penetrating just 1 mm of the stone. Further annealing of these stones (Temperature range being 500-900°C for neutron-bombarded stones and from 500-1200°C for electron-bombarded stones) gives out orange, yellow, brown, or pink colors. Blue to blue-green gems that are not annealed are separated from natural stones in the same manner as gamma ray-treated stones.
Before annealing, almost all irradiated diamonds exhibit a characteristic "absorption spectrum" consisting of a fine range in the far red, at 741 nm known as the GR1 line. This is usually considered as a strong indication of treatment. Annealing usually destroys this property, but generally creates several others. The most persistent of these resides at 595 nm. However, if the annealing is performed above 1000°C, the 595 line is also destroyed, but leaves two new lines at 1936 and 2024 nm present in the infrared. All these lines can be detected in gemological laboratories using good quality spectrophotometers. These lines are best detected when the stone is kept at temperatures below -150°C.
But some irradiated diamonds are completely natural. The famous example is Dresden Green Diamond. These natural stones get their color imparted by "radiation burns" in the form of small patches, usually only at the surface, similar to radium-treated diamonds. Naturally irradiated diamonds also possess the GR1 line.
During the Georgian and Victorian era, colored metal foil coatings were applied to the pavilion of diamonds as was the first treatment applied to diamonds after cutting and polishing. These diamonds were placed in closed-back jewelry settings. Under magnification, there were many flaws visible such as flaking of foil, air bubbles between the diamond's surface and the foil and moisture between the surfaces causing uneven color. In spite of these flaws, the jewelry consisting of these diamonds still holds value due to the antique status.
Today, though more sophisticated methods of coating diamonds have been devised. These include Violet blue dyes and vacuum-sputtered films. These coatings effectively whiten the apparent color of a yellow tinted diamond and is usually applied to the pavilion or girdle of a diamond. These coatings are hard to detect. Dyes can be removed with hot water or alcohol with ease whereas vacuum sputtered films require sulphuric acid for their removal. Although under high magnification trapped air bubbles and scratched layers of coatings are visible. These treatments are considered fraudulent unless disclosed.
Another method of coating includes application of synthetic diamond layer on the diamond's stimulant. This offers resistance to scratches, higher thermal conductivity and lower electrical conductivity. These treatments are even more difficult to detect through conventional methods and are therefore considered fraudulent if attempted to present a simulated diamond as real.
Few gem-quality stones that possess a brownish body color can be treated by HTHP method to get their color significantly lightened or altogether removed. This method was introduced by General Electric in 1999. Diamonds treated by this method are of Type Iia. The color of these diamonds results from structural defects that arose during crystal growth, known as plastic deformations. HTHP treatment is believed to repair the crystal deformations or according to other researchers, HTHP destroys stable vacancy clusters and therefore whitens the stone. Type Ia diamonds having nitrogen impurities present in clusters. These impurities do not normally affect body color, but can be treated by HTHP for color alteration. HTHP treatment is also given to some synthetic diamonds alter their optical properties and make them look like good quality natural diamonds. Pressure of up to 70,000 atmospheres and temperature of up to 2,000°C is generally used in HTHP treatment. Unequivocal identification of HTHP stones is performed by renowned gemological laboratories. Fourier transform spectroscopy (FTIR) and Raman spectroscopy are mostly used to analyze the visible and infrared absorption of suspected diamonds in order to detect characteristic absorption lines, some of them can be the ones indicative of exposure to high temperatures. Under the microscope, indicative features observed are internal graining (Type IIa), partially healed feathers, hazy appearance, black cracks surrounding inclusions, or a beaded or frosted girdle. Diamonds that have been treated for color enhancements by General Electric are given laser inscriptions on their girdle. These inscriptions can easily be polished and removed. Therefore, HTHP treatment should be disclosed to the buyer.