A structurally pure and perfect diamond is completely transparent and colorless.
But, in reality, almost no natural gemstones are perfect, and therefore have defects and impurities which give them a color varying from barely noticeable to bold and intense.
Color is one of the four Cs of diamond quality, and it is graded in a similar way that clarity is graded. Depending on the hue and intensity, coloration can either add to or detract from a diamond's value.
Diamonds are found in a wide variety of colors, including steel gray, white, orange, blue, yellow, green, red, pink, purple, brown, and black.
The stones are classed into two main types, I and II, and various subtypes, depending on the nature of the impurities and how they affect the way light is absorbed.
Type I diamonds have nitrogen atoms as the main impurity, and normally have a yellow or brown tint as a result.
Type II diamonds have no measurable nitrogen impurities, but coloration occurs due to plastic deformation during crystal growth. These diamonds tend to be colored pink, red, or brown, and are much rarer – accounting for just 1.8% of gem diamonds.
Color grading depends on whether a diamond falls into the normal color range, or has an intense enough hue to be considered to be a fancy color diamond.
Diamond color is graded on a scale which ranges from D – totally colorless – to Z, which is a pale yellow or brown. Brown diamonds darker than K are usually described using their letter grade, and a descriptive phrase, for example M Faint Brown, while diamonds with a deeper color than Z fall into the fancy color range.
When a diamond's color is graded, it is compared to a master stone set of diamonds. Each master stone exhibits the least amount of color that a diamond in that grade may exhibit, and an expert grader uses them to assess where in the range the color of the sample diamond falls.
Grading laboratories normally have a complete set of master stones for every color grade, but an independent grader working in a retail environment might only have a smaller subset of master stones covering just the typical grade range of color they are expecting to encounter.
The most commonly-used color grading system, used by the Gemological Institute of America (GIA), works as follows:
D – F: Colorless
G – J: Near Colorless
K – M: Faint Yellow
N – R: Very Light Yellow
S – Z: Light Yellow
Fancy color diamonds are graded using different systems which indicate the characteristics rather than just the presence of a color – similar to the way rubies, sapphires and emeralds are graded.
Laboratories use a list of 27 color hues that span the full spectrum for colored gems and diamonds: (Red, Orangish-Red, Reddish-Orange, orange, Yellowish-Orange, Yellow-Orange, Orange-Yellow, Orangish-Yellow, Yellow, Greenish-Yellow, Green-Yellow, Yellow-Green, Yellowish-Green, Green, Bluish-Green, Blue-Green, Green-Blue, Greenish-Blue, Blue, Violetish-Blue, Bluish-Violet, Violet, Purple, Reddish-Purple, Red-Purple, Purple-Red, Purplish-Red).
Other combination colors and descriptors can be added, including Fancy Intense, Fancy Deep and Fancy Vivid. The terms Champagne, Cognac and Coffee can be used to describe different types of brown diamonds.
The color of a diamond can be altered using three main treatments: irradiation, heat and pressure treatment, or the application of thin films or coatings.
Only heat and pressure treatment can actually remove color from rough or cut diamonds – the other two methods can only modify color. The method of applying film or coating is also not considered to be a permanent enhancement, as the film can wear away over time.
Diamonds rated D or E (colorless) are much more rarer than those values, R, S or Y (tinted).
This is partly because diamonds with less of a tint are naturally rarer, thus limiting supply, but also because the bright white appearance of a colorless diamond is more desired by consumers.
Fancy color diamonds affect value differently. If the color is rare, value increases with the intensity of the color. Fashion trends also play a role – for example, pink diamonds rose in price after Jennifer Lopez received a pink diamond engagement ring.
The deep blue Hope Diamond is among the most valuable in the world. In 2009 a 7-carat (1.4 g) blue diamond was sold at auction for 10.5 million Swiss francs (US$9.5 million at the time).
Because diamond color varies so widely, buyers make decisions based on taste and budget.
Purists should obviously look for colorless diamonds with a grade of D – F. But those wanting clear, white diamonds can find much better value in the Near Colorless range, where stones show no noticeable color to the naked eye.