Green diamonds are among the rarest of all fancy colors.
The most famous and largest natural green diamond is the 41 carat Dresden Green.
The Dresden Green is an exceptional diamond, because its green color is evenly distributed throughout the body of the stone. While most green diamonds owe their color to surface "skins," the Dresden is uniformly green.
Cause of color in green diamonds is natural radiation, which creates an external skin or surface of green color. It is uncommon for the green color to reach deep within a diamond. Lab graders assess the natural color of green diamonds by this skin. Irradiated green diamonds are often found in the marketplace, so determining natural color for value is key. Also note that sometimes labs are unable to conclusively determine the natural origin of green diamonds because the cause is radiation. So it is not unusual for a manufacturer to submit a rough diamond or partially polished diamond so that the lab can identify the natural cause of color prior to completion of polishing.
Green is the color between blue and yellow, so common secondary colors in green diamonds include bluish green and yellowish green, and if the color is desaturated, grayish green or brownish green.
Natural green diamonds are second to red diamonds as the world's rarest fancy color. True greens devoid of secondary hue are the most valuable among natural green diamonds.
Green diamonds are graded as Faint Green, Very Light Green, Light Green, Fancy Light Green, Fancy Green, Fancy Intense Green, Fancy Dark Green, Fancy Deep Green, and Fancy Vivid Green.