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Colored Diamonds,
While most diamonds are in the colorless to light yellow range, some have a natural color that is deep, distinct, and opulent. These are known as fancy-color diamonds and are often blue, brown, or pink. Unlike colorless and near-colorless diamonds which are valued for their lack of color, fancies are valued for the intensity of their color. Colored diamonds are a small but increasingly popular segment of the diamond market.

The physical conditions necessary to color a diamond naturally occur very seldom, making natural color diamonds extremely rare. For every natural color diamond, there are 10,000 colorless ones that have made the trip from the earth’s depths to its surface. It is this entirely natural process of geographical formation which ensures that each natural color diamond is one of a kind.

The formation of natural color diamonds is a process that requires the presence of additional trace elements and distortions to the typical diamond crystal. During the creation of a diamond, if an element interacts with its carbon atoms, the color can change. Natural radiation and pressure on a diamond’s structure can also intensify its color.

Rather than emphasizing the brilliance and fire coveted in near-colorless diamonds, these stones are all about the color intensity. The Argyle mine in Western Australia launched a massive marketing campaign some time ago that helped change the public’s perception of these previously overlooked diamonds. The 1987 sale of the Hancock Red, at a record auction price of $926,000 per carat, further magnified the allure of fancies.
Color Grading of Fancy Diamonds
GIA’s system for color-grading colored diamonds was developed in the mid-1950s and revamped in the mid-1990s. The diamond color grading system expresses color using the attributes of hue (the characteristic color), tone (the color’s relative lightness or darkness), and saturation (the strength or weakness of the color). Using controlled viewing conditions and color comparators, the grader determines the stone’s color from one of 27 hues. The fancy grade describes the stone’s tone and saturation with romantic names like “Fancy Light,” “Fancy Intense,” and “Fancy Vivid.”

Today, the GIA color grading system for colored diamonds is used worldwide. Many of the most famous colored diamonds, including the Blue Hope, the Dresden Green, and the Hancock Red, have been examined by the GIA laboratory using GIA’s color grading system.

GIA offers two types of diamond grading reports for colored diamonds. The GIA Colored Diamond Grading Report contains the same comprehensive diamond information as the GIA Diamond Grading Report. In addition, the GIA Colored Diamond Identification and Origin Report, known as the color-only report, gives a color grade and the nature of the color.
Colored Gemstones
While renowned for its diamond grading expertise, the GIA Laboratory also receives a vast array of colored gemstones for identification—and its work in this area has been equally remarkable. Over the decades, the Institute has created a database of information on more than 100,000 individual gemstones. Using this database and sophisticated analytical tools, GIA researchers can pinpoint a gem’s identity, and depending on the gemstone, even it's geographic orgin. They also distinguish synthetics, simulants, and stones that have undergone treatment. A particularly important activity involves determining the origin of color in gemstones—whether it is natural or the result of a treatment process.

This right up is from the GIA web site.