Diamond color
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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. The Aurora Diamond Collection displays a spectacular array of naturally colored diamonds, including the red color diamonds.
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[edit] 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 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[citation needed] 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 discernible visible absorption spectrum.
[edit] Grading White Diamonds
The majority of diamonds that are mined are in a range of pale yellow or brown color that is termed the normal color range. Diamonds that are of intense yellow or brown, or any other color are called fancy color diamonds. Diamonds that are of the very highest purity are totally colorless, and appear a bright white. The degree to which diamonds exhibit body color is one of the four value factors by which diamonds are assessed.
[edit] History of color grading
Color grading of diamonds was performed as a step of sorting rough diamonds for sale by the London Diamond Syndicate.
As the diamond trade developed, early diamond grades were introduced by various parties in the diamond trade. Without any co-operative development these early grading systems lacked standard nomenclature, and consistency. Some early grading scales were; I, II, III; A, AA, AAA; A, B, C. Numerous terms developed to describe diamonds of particular colors: golconda, river, jagers, cape, blue white, fine white, and gem blue.
[edit] Grading the normal color range
The Gemological Institute of America (GIA) in the 1950's developed a grading scale for diamonds in the normal color range. The scale ranges from D which is totally colorless to Z which is a pale yellow or brown color. Brown diamonds darker than K color are described using their letter grade, and a descriptive phrase. Diamonds between K and M are graded with the phrase Faint Brown, N to R are Very Light Brown, and S to Z are graded Light Brown. A brown S color diamond would then be graded S-Light brown. Diamonds with more depth of color than Z color fall into the fancy color diamond range.
| GIA | Grade | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ||||||||||||||||||||||||||||
| Description | Colorless | Near Colorless | Faint Yellow | Very Light Yellow | Light Yellow | |||||||||||||||||||||||||||||||||||||||||||||||
Diamond color is graded by comparing a sample stone to a masterstone set of diamonds. Each masterstone is known to exhibit the very least amount of body color that a diamond in that color grade may exhibit. When sample stones are compared with the master stone, the grader assesses whether the sample has more, less or equal color to the masterstones. A grading laboratory will possess a complete set of masterstones representing every color grade. However, the independent grader working in a retail, or appraisal service setting will possess a range of masterstones that covers only the typical grade range of color they expect to encounter while grading. A typical grading set of masterstones would consist of five diamonds in two grade increments, such as an E, G, I, K, and M. It is not common for a grader to possess a D masterstone, as the E masterstone is more useful in dividing the D and E color grades. The intermediate grades are assessed by the graders judgement.
Diamonds in the normal color range are graded loose, with the table facet facing downward and pavillion side upwards.
| GIA | Status: current | AGS | Status: current | AGS | Status: historical: pre 1995 | CIBJO Status: current | IDC | Status: current | Scan. D.N. | Status: current | Old World Terms | Status: historical |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| grade and description[2] | grade and electronic colorimeter scale[3] | grade and electronic colorimeter scale[3] | grade[4] | grade and description[4] | grade for .50ct and over[5] | grade for under .50ct | series 1 scale[4] | series 2 scale[4] | ||||
| D | Colorless | 0 | 0 - 0.49 | 0 | 0 - 0.75 | Exceptional white + | Exceptional white + | Colorless | River | White | Finest White | Jager |
| E | 0.5 | 0.5 - 0.99 | Exceptional white | Exceptional white | River | |||||||
| 1 | 0.76 - 1.35 | |||||||||||
| F | 1.0 | 1.0 - 1.49 | Rare white + | Rare white + | Colorless when viewed through the crown | Top Wesselton | Fine White | |||||
| 2 | 1.36 - 2.00 | |||||||||||
| G | Near Colorless | 1.5 | 1.5 - 1.99 | Rare white | Rare white | Top Wesselton | ||||||
| H | 2.0 | 2.0 - 2.49 | 3 | 2.01 - 2.50 | White | White | Wesselton | White | Wesselton | |||
| I | 2.5 | 2.5 - 2.99 | 4 | 2.51 - 3.0 | Slightly tinted white | Slightly tinted white | Slightly colored | Top Crystal | Slightly tinted white | Commercial White | Top Crystal | |
| J | 3.0 | 3.0 - 3.49 | 5 | 3.01 - 3.75 | Crystal | Top silver cape | Crystal | |||||
| K | Faint Yellow | 3.5 | 3.5 - 3.99 | Tinted white | Tinted white | Top cape | Tinted white | Top cape | ||||
| 6 | 3.76 - 4.5 | Silver cape | ||||||||||
| L | 4.0 | 4.0 - 4.49 | ||||||||||
| M | 4.5 | 4.5 - 4.99 | 7 | 4.51 - 5.50 | Tinted color 1 | Tinted color | Slightly colored to colored | Cape | Tinted color | Light cape | Cape | |
| N | Very Light Yellow | 5.0 | 5.0 - 5.49 | Tinted color 2 | Low Cape | |||||||
| O | 5.5 | 5.5 - 5.99 | 8 | 5.51 - 7.0 | Light yellow | Cape | Very light yellow | |||||
| P | 6.0 | 6.0 - 6.49 | Light yellow | |||||||||
| Q | 6.5 | 6.5 - 6.99 | ||||||||||
| R | 7.0 | 7.0 - 7.49 | 9 | 7.01 - 8.5 | ||||||||
| Dark cape | ||||||||||||
| S | Light Yellow | 7.5 | 7.5 - 7.99 | Tinted color 3 | Yellow | |||||||
| T | 8.0 | 8.0 - 8.49 | ||||||||||
| U | 8.5 | 8.5 - 8.99 | 10 | 8.51 - 10.00 | ||||||||
| V | 9.0 | 9.0 - 9.49 | ||||||||||
| W | 9.5 | 9.5 - 9.99 | ||||||||||
| X | 10.0 | 10 + | 10+ | |||||||||
| Y | ||||||||||||
| Z | ||||||||||||
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The European Gemological Laboratory (USA) (EGLUSA) also uses the color scale developed by the GIA.
"D" color has a unique "icy" look to it. 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.
[edit] Grading fancy color diamonds
Yellow or brown color diamonds having color more intense than "Z", as well as diamonds exhibitng color other than yellow or brown are considered fancy colored diamonds. These diamonds are graded using separate systems which indicate the characteristics of the color, and not just its presence. These color grading systems are similar to those used for other colored gemstones, such as ruby, sapphire, or emerald, than they are to the system used for white diamonds.
[edit] GIA colored diamond grading system
The GIA issues grading a Colored Diamond Grading Report for colors that are not in the normal color range of diamonds. The GIA uses a list of 27 color hues that span the full spectrum of colors. The tone and saturation of these hues are then described with one of nine descriptors; Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Dark, Fancy Deep, Fancy Vivid.
[edit] Gran Colorimeter
Color can also be determined using a device called the Gran Colorimeter, manufactured by Sarin Technologies. It measures from D to Z to Fancy Intense with an accuracy within ±½ of a color grade on loose stones from 0.25 to 10 carats (as low as .15 carat or as high as 20 carats with reduced accuracy), and you can specify which grading scale it should use (GIA, GEM, IGI, AGS, HRD, and others). The accuracy is within ±1 color grade for mounted stones. If you diamond is a "G" color it will tell you whether it's a "high G" or a "low G."
The Gran colorimeter was first developed by Paul Gran in 1972 at Gran Computer Industries Ltd.
[edit] 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. The Aurora Diamond Collection of natural color diamonds is one of the most comprehensive diamond collections in the world.
[edit] See also
[edit] External links
[edit] References
- Haske, Martin. GIA GTL's Color Grading Of Fluorescent Diamonds. (Retrieved March 15, 2005.)
