The Gemological Institute of America (GIA), the American Gem Society (AGS), and the International Gemological Institute (IGI) are the three most widely known and respected diamond grading laboratories in the world. Each of the grading laboratories have developed a very similar nomenclature for identifying the 4 C’s of diamonds.
D to Z
Color grading scales used by the internationally recognized laboratories (GIA & IGI for example), ranges from totally colorless (D) to pale yellow or brown color (Z). 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.
The coloration of diamonds can be caused by several factors. Impurities trapped in the diamond during its formation, the crystal lattice structure of the diamond, and the exposure to radiation can all lead to the wide verity of colors available in diamonds.
Yellow Diamonds
Yellow diamonds are colored because of the impurities that are trapped inside diamonds when they are created. If a few of the millions of carbon atoms have been replaced by nitrogen atoms, then structure of the diamond will not be significantly altered but the clarity will be changed. The amount of color displayed is dependent on the amount of nitrogen involved.
When we see color it is because the object we are looking at reflected a specific wavelength of the light spectrum. A good example of this is a yellow flower. The flower absorbs all of the light except the yellow light, which is reflected by the yellow flower.
Blue Heart Diamond
In the case of a blue colored diamond some of the nitrogen has been replaced by Boron. Boron will reflect the blue wavelength of the light spectrum. The higher the concentration of boron the more color will be showed. At a level of one or a few boron atoms for every million-carbon atom, an attractive blue color results.
Graff Pink Supreme Diamond
The pink diamonds comes in shades ranging from a pastel rose, such as the Pink Orchid to intense purple-reds of the Moussaieff Red, and the price is determined by the intensity of the color. Pink diamonds have sold for up to $1,000,000 a carat. Unlike the Type I diamonds that derived their color from impurities embedded in the diamond, Pink diamonds are considered a Type II and get their color from a process known as Plastic Deformation.
Type II diamonds have very few if any nitrogen impurities in them. They get their coloration due to structural anomalies caused by Plastic Deformation during the crystal growth. The intense pressure changes the lattice structure of diamonds and has led to the formation of Pink, and Red colored diamonds.
Green Diamond
A natural diamond coming into contact with a radioactive source at some point during its lifetime causes some diamonds to develop a green coloration. The time required may be as much as a million years or longer. Green diamonds of this nature are very unique.
The most common form of irradiation diamonds comes from alpha particles found in uranium compounds or from percolating groundwater. Green spots on the surface of the diamonds or a thin green film may develop on the skin of the diamond after long exposure to these particles. Many times this green coloration will be removed during the cutting or faceting process.
Bombardment by beta and gamma rays will color the diamond to a greater depth and in some rare case turn the entire stone green. Heating to temperature to just below 600 degrees Celsius can sometimes also cause a diamond to develop a green ting. Higher temperature may turn the stone to a less desirable yellow or brown color.
Black Diamond
Black diamonds are found only in Brazil and the Central African Republic. Approximately 600 tons of conventional diamonds have been mined, traded, polished since 1900. But not a single black diamond has been discovered in the world’s mining fields. The geological settings where diamonds are found or mined are virtually identical with the one exception, the Black Diamond.
Black diamonds have been found to contain trace elements of nitrogen and hydrogen. The study published in 2006 analyzed the hydrogen in black diamond samples using infrared-detection instruments and found that the quantity indicated that the mineral formed in a supernova explosion prior to the formation of the Solar System. These diamonds were formed by carbon-rich cosmic dust in an environment near carbon stars. The diamonds were incorporated into solid bodies that subsequently fell to Earth as meteorites.