Diamonds with increasing shades of brown and yellow are referred to as champagne diamonds, and diamonds with exceptional color, such as pink, red, green, blue, and amber, are called "Fancies." With the exception of some natural fancy colors, such as blue, pink, purple, or red, the colorless grade is the most rare. Colors of high saturation such as red and green, which have no modifying secondary colors, are very rare indeed and command world record prices.
The rarest and most prized diamonds are those with no hint of color at all whose beauty is entirely dependent upon their optical properties. Such stones reflect all of the colors of the rainbow, and their value is great. All things being equal, other than the fancy colors, the closer a diamond is to colorless, the more valuable it is.
The color of the stone can be determined by using a GIA (Gemological Institute of America) Certified set of master stones and/or the Colorimeter, a computer which accurately grades the color of a polished diamond.
Factors Affecting Color
Other factors which can affect color include fluorescence, color enhancements such as high pressure, high temperature treatment (HPHT) and irradiation.
Fluorescence
produced by ultraviolet light from the sun, by black lighting or other long-wavelength UV source, occurs in an estimated 35% of gem grade diamonds. (Virtually all diamonds fluoresce when exposed to X-rays, and this forms the basis for their identification and collection at mining sites.) The UV light excites electrons in the diamond crystal, which then release this absorbed energy in the form of visible light, producing a blue, or sometimes other color, of faint to very strong intensity. Once the light source is removed however, the fluorescence is no longer observed. If, in rare situations, light emission continues for a period after the exciting light has been turned off, the phenomenon is called phosphorescence.
