In nature, diamonds come in different colors. The diamonds that range from colorless to light yellow fall within the normal color range.
Two diamonds of the same clarity, weight, and cut can differ in value based on color alone. Within the normal color range, colorless diamonds are the rarest, so they're the most valuable. Even the slightest hint of color can make a dramatic difference in value. The normal color grading scale that was developed by Richard T. Liddicoat at GIA (Gemological Institute of America) in the 1950s, describes diamond colors from D (colorless) to Z (light yellow). Today, the GIA Color Scale is the most widely used diamond color-grading system in the world.
These diamonds – graded E, K, and Z – represent diamond colors that are near the top, middle, and bottom of the GIA Color Scale.
Diamonds that fall outside the normal color range are often called “fancy-colored diamonds”. This group includes diamonds that are yellow and have more color than a Z color grade, and all diamonds that exhibit a color other than yellow.
Unlike the diamonds falling within the normal color range, fancy-colored diamonds increase in their value as the color deepens. In nature, diamonds come in almost any color. Red, green, purple, and orange are generally the rarest, followed by pink and blue. Yellows and browns are the most common colors. Many fancy colors, however, are not strong and pure. They're often blended with other colors and muted by grayishness or brownishness.
Large, vivid, fancy-colored diamonds are extremely rare. They're also very valuable. In November 2013 at Sotheby's auction house, Geneva, this 59.6-carat pink diamond was sold for an incredible 76.3 million Swiss francs.
Some diamonds emit a visible light called fluorescence when they're exposed to ultraviolet (UV) radiation. Under the right conditions, you can see fluorescence in about one third of gem-quality diamonds. Blue is the most common fluorescent color, but other possible fluorescent colors include white, yellow, and orange.
In some diamonds with J to Z color in the normal color grade, fluorescence may cause a light yellow diamond look closer to colorless in sunlight. This is a positive effect for the gemstone value. But if the fluorescence in close to colorless diamonds is too strong it might make the stone look cloudy or “oily” and that can lower the value of the diamond.
Diamonds with various degrees of fluorescence. No difference is observed under normal lighting conditions. The fluorescence degree will only show under long-wave ultraviolet light.