A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice. Depending on the hue and intensity of a diamond's coloration, a diamond's color can either detract from or enhance its value. For example, most white diamonds are discounted in price when more yellow hue is detectable, while intense pink or blue diamonds (such as the Hope Diamond) can be dramatically more valuable. Out of all colored diamonds, red diamonds are the rarest of all.

Possible colors

Diamonds occur in a restricted variety of colors — steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black. Colored diamonds contain interstitial impurities or structural defects that cause the coloration, whilst pure diamonds (which are exceedingly rare) are perfectly transparent and colorless. Diamonds are scientifically classed into two main types and several subtypes, according to the nature of impurities present and how these impurities affect light absorption:

Type I diamond has nitrogen (N) atoms as the main impurity, commonly at a concentration of 0.1 percent. If the N atoms are in pairs they do not affect the diamond's color; these are Type IaA. If the N atoms are in large even-numbered aggregates they impart a yellow to brown tint (Type IaB). About 98 percent of gem diamonds are type Ia, and most of these are a mixture of IaA and IaB material: these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. If the N atoms are dispersed throughout the crystal in isolated sites (not paired or grouped), they give the stone an intense yellow or occasionally brown tint (Type Ib); the rare canary diamonds belong to this type, which represents only 0.1 percent of known natural diamonds. Synthetic diamond containing nitrogen is Type Ib. Type I diamonds absorb in both the infrared and ultraviolet region, from 320 nm. They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties of diamond).

Type II diamonds have very few if any nitrogen impurities. Type IIa diamond can be colored pink, red, or brown due to structural anomalies arising through plastic deformation during crystal growth—these diamonds are rare (1.8 percent of gem diamonds), but constitute a large percentage of Australian production. Type IIb diamonds, which account for 0.1 percent of gem diamonds, are usually a steely blue or grey due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types (see Electrical properties of diamond). However, an overabundance of hydrogen can also impart a blue color; these are not necessarily Type IIb. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225 nm, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernable visible absorption spectrum.

Color intensity scale

The Gemological Institute of America (GIA) uses a "D" to "Z" scale it developed for grading the color of "white" (non-fancy colored) diamonds, where "D" is colorless and "Z" is yellow:

• colorless: D, E, F
• near colorless: G, H, I, J
• faint yellow or brown: K, L, M
• very light yellow or brown: N, O, P, Q, R
• light yellow or brown: S, T, U, V, W, X, Y, Z

Other diamond grading agencies, notably the European Gemological Laboratory (EGL) and the American Gemological Society (AGS), also use the color scale developed by the GIA.

Diamonds that rate toward the colorless end of the range are sometimes known as "high-color" diamonds, and those toward the other end, "low-color" diamonds. These terms refer to the relative desirability (as demonstrated by market prices) of color grades, not the intensity of the color itself.

Diamond color is graded by comparing a sample stone to a master set of diamonds with colors defined as "D" through "Z". Diamonds having a color more intense than "Z" are considered fancy colored diamonds, and can be graded using separate systems which indicate the characteristics of the color, and not just its presence. These color grading systems are more similar to those used for other colored gemstones, such as ruby, sapphire, or emerald, than they are to the system used for white diamonds.

There is no scientific standard for grading the color diamonds. While the GIA may grade a diamond's color an "I", the EGL may grade the same diamond a "H", and a jeweler who has not had the diamond certified may claim it to be a "G".

Value of colored diamonds

Diamonds that enter the Gemological Institute of America's scale are valued according to their clarity. For example, an "E" rated diamond (colorless) is much more valuable than a "Y" rated diamond (light yellow or brown). This is due to two effects: high-color diamonds are rarer, limiting supply; and the bright white appearance of high-color diamonds is more desired by consumers, increasing demand. Poor color is usually not enough to eliminate the use of diamond as a gemstone: if other gemological characteristics of a stone are good, a low-color diamond can remain more valuable as a gem diamond than an industrial-use diamond.

Diamonds that go out of scale in the rating are known as "fancy color" diamonds. These diamonds are valued using different criteria than those used for regular diamonds. When the color is rare, the more intensely colored a diamond is, the more valuable it becomes. Fancy-colored diamonds such as the deep blue Hope Diamond are among the most valuable and sought-after diamonds in the world.

See also

• Diamond
• Diamond enhancement

External links

• Certain diamonds can be colored to more desirable colors by irradiation or HPHT

References

• Haske, Martin. GIA GTL's Color Grading Of Fluorescent Diamonds. (Retrieved March 15, 2005.)