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. The Aurora Diamond Collection displays a spectacular array of naturally colored diamonds, including red color diamonds.
Possible colors
Main article: Diamond typeDiamonds occur in a variety of colors — steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black, virtually in every color of the rainbow. 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 atoms as the main impurity, commonly at a concentration of 0.1%. If the nitrogen atoms are in pairs they do not affect the diamond's color; these are Type IaA. If the nitrogen atoms are in large even-numbered aggregates they impart a yellow to brown tint (Type IaB). About 98% 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 nitrogen 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% 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 no measurable nitrogen impurities. 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. 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% of gem diamonds), but constitute a large percentage of Australian production. Type IIb diamonds, which account for 0.1% of gem diamonds, are usually light blue due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types (see Electrical properties of diamond). However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron. Also not restricted to type are green diamonds, whose color is derived from exposure to varying quantities of radiation.
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.
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 , "brown".
Grading the normal color range
Refers to a grading scale for diamonds in the normal color range used by internationally recognized laboratories (GIA & IGI for example). 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 usually described using their letter grade, and a descriptive phrase, for example M Faint Brown . Diamonds with more depth of color than Z color fall into the fancy color diamond range.
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 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 pavilion side upwards. When color grading is done in the mounting, other techniques will apply and the grade will usually be expressed as a range (for example F-G)
"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.
Grading fancy color diamonds
Yellow or brown color diamonds having color more intense than "Z", as well as diamonds exhibiting 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 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.
Colored diamond grading system
It refers to the color grading system used by internationally recognized laboratories (GIA and IGI for example) for colors that are not in the normal color range of diamonds. These laboratories 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 Vivid
- Fancy Deep
- Fancy Dark .
The grading has an additional aspect that describes how well is the color applicable on the diamond.
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 (50 to 2,000 mg) (as low as 0.15 carat (30 mg) or as high as 20 carats (4 g) 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 your diamond is a "G" color it will tell you whether it is a "high G" or a "low G."
The Gran colorimeter was first developed by Paul Gran in 1972 at Gran Computer Industries Ltd.
Value of colored diamonds
The Darya-I-Nur Diamond is one of the world's largest diamonds and is one of the most rare because of its pale pin