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.
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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.
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. The emission of alpha particles by the radium was responsible. Unfortunately radium treatment also left the diamond strongly radioactive, to the point of being unwearable. A diamond octahedron so treated was donated by Crookes to the British Museum in 1914, where it remains today: it has not lost its color nor its radioactivity.
Nowadays diamond is safely irradiated in four ways: proton and deuteron bombardment via cyclotrons; gamma ray bombardment via exposure to cobalt-60; neutron bombardment via the piles of nuclear reactors; and electron bombardment via Van de Graaff generators. These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing color centers. Irradiated diamonds are all some shade of green, black, or blue after treatment, but most are annealed to further modify their color into bright shades of yellow, orange, brown, or pink. The annealing process increases the mobility of individual carbon atoms, allowing some of the lattice defects created during irradiation to be corrected. The final color is dependent on the diamond's composition and the temperature and length of annealing.
Cyclotroned diamonds have a green to blue-green color confined to the surface layer: they are later annealed to 800°C to produce a yellow or orange color. They remain radioactive for only a few hours after treatment, and due to the directional nature of the treatment and the cut of the stones, the color is imparted in discrete zones. If the stone was cyclotroned through the pavilion (back), a characteristic "umbrella" of darker color will be seen through the crown (top) of the stone. If the stone was cyclotroned through the crown, a dark ring is seen around the girdle (rim). Stones treated from the side will have one half colored deeper than the other. Cyclotron treatment is now uncommon.
Gamma ray treatment is also uncommon, because although it is the safest and cheapest irradiation method, successful treatment can take several months. The color produced is a blue to blue-green which penetrates the whole stone. Such diamonds are not annealed. The blue color can sometimes approach that of natural Type IIb diamonds, but the two are distinguished by the latter's semiconductive properties. As with most irradiated diamonds, most gamma ray-treated diamonds were originally tinted yellow; the blue is usually modified by this tint, resulting in a perceptible greenish cast.
The two most common irradiation methods are neutron and electron bombardment. The former treatment produces a green to black color that penetrates the whole stone, while the latter treatment produces a blue, blue-green, or green color that only penetrates about 1 millimeter deep. Annealing of these stones (from 500–900°C for neutron-bombarded stones and from 500–1200°C for electron-bombarded stones) produces orange, yellow, brown, or pink. Blue to blue-green stones that are not annealed are separated from natural stones in the same manner as gamma ray-treated stones.
Prior to annealing, nearly all irradiated diamonds possess a characteristic absorption spectrum consisting of a fine line in the far red, at 741 nm — this is known as the GR1 line and is usually considered a strong indication of treatment. Subsequent annealing usually destroys this line, but creates several new ones; the most persistent of these is at 595 nm.
It should be noted that some irradiated diamonds are completely natural. One famous example is the Dresden Green Diamond. In these natural stones the color is imparted by "radiation burns" in the form of small patches, usually only skin deep, as is the case in radium-treated diamonds. Naturally irradiated diamonds also possess the GR1 line.
The largest known irradiated diamond is the Deepdene.
The application of colored tinfoil to the pavilion (back) surfaces of gemstones was common practice during the Georgian and Victorian era; this was the first treatment — aside from cutting and polishing — applied to diamond. Foiled diamonds are mounted in closed-back jewelry settings, which may make their detection problematic. Under magnification, areas where the foil has flaked or lifted away are often seen; moisture that has entered between the stone and foil will also cause degradation and uneven color. Because of its antique status, the presence of foiled diamonds in older jewelry will not detract from its value.
In modern times, more sophisticated surface coatings have been developed: these include violet-blue dyes and vacuum-sputtered films resembling the magnesium fluoride coating on camera lenses. These coatings effectively whiten the apparent color of a yellow-tinted diamond, because the two colors are complementary and act to cancel each other out. Usually only applied to the pavilion or girdle region of a diamond, these coatings are among the hardest treatments to detect — while the dyes may be removed in hot water or alcohol with ease, the vacuum-sputtered films require a dip in sulfuric acid to remove. The films can be detected under high magnification by the presence of raised areas where air bubbles are trapped, and by worn areas where the coating has been scratched off. These treatments are considered fraudulent unless disclosed.
Another coating treatment applies a thin film of synthetic diamond to the surface of a diamond simulant. This gives the simulated diamond certain characteristics of real diamond, including higher resistance to wear and scratching, higher thermal conductivity, and lower electrical conductivity. While resistance to wear is a legitimate goal of this technique, some employ it in order to make diamond simulants more difficult to detect through conventional means, which may be fraudulent if they are attempting to represent a simulated diamond as real.
A small number of otherwise gem-quality stones that possess a brown body color can have their color significantly lightened or altogether removed by HTHP treatment, or, depending on the type of diamond, improve existing color to a more desirable saturation. The process was introduced by General Electric in 1999. Diamonds treated to become colorless are all Type IIa and owe their marring color to structural defects that arose during crystal growth, known as plastic deformations, rather than to interstitial nitrogen impurities as is the case in most diamonds with brown color. HTHP treatment is believed to repair these deformations, and thus whiten the stone. (This is probably an incorrect conclusion, the whitening due to destruction of stable vacancy clusters according to one of the researchers). Type Ia diamonds, which have nitrogen impurities present in clusters that do not normally affect body color, can also have their color altered by HTHP. Some synthetic diamonds have also been given HTHP treatment to alter their optical properties and thus make them harder to differentiate from natural diamonds. Pressures of up to 70,000 atmospheres and temperatures of up to 2,000°C (3,632°F) are used in HTHP.
Also in 1999, Novatek, a Provo, UT manufacturer of industrial diamonds known for its advancements in diamond synthesis, accidentally discovered that the color of diamonds could be changed by the HTHP process. The company formed NovaDiamond, Inc. to market the process. By applying heat and pressure to natural stones, NovaDiamond could turn brown Type I diamonds light yellow, greenish yellow, or yellowish green; improve yellowish Type IIa diamonds by several color grades, even to white; intensify the color of yellow Type I diamonds; and make some bluish gray Type I and Type IIb colorless (although in some cases natural bluish gray diamonds are more valuable left alone, as blue is a highly desired hue). In 2001, however, NovaDiamond quit the HTHP gem business because of what the company's leader, David Hall, characterized as the underhanded practices of dealers. Apparently, dealers were passing off NovaDiamond enhanced gems as naturally colored, and the company refused to be party to this deception.
Definitive identification of HTHP stones is left to well-equipped gemological laboratories, where Fourier transform spectroscopy (FTIR) and Raman spectroscopy are used to analyze the visible and infrared absorption of suspect diamonds to detect characteristic absorption lines, such as those indicative of exposure to high temperatures. Indicative features seen under the microscope include: internal graining (Type IIa); partially healed feathers; a hazy appearance; black cracks surrounding inclusions; and a beaded or frosted girdle. Diamonds treated to remove their color by General Electric are given laser inscriptions on their girdles: these inscriptions read "GE POL", with "POL" standing for Pegasus Overseas Ltd, a partnered firm. It is possible to polish this inscription away, so its absence cannot be a trusted sign of natural color. Although it is permanent, HTHP treatment should be disclosed to the buyer at the time of sale.