Most commercially available diamonds are classified by color, or more appropriately, the lack of color. The most valuable diamonds are those classified as colorless, yet there are stones that have rich colors inluding yellow, red, green and even black that are extremely rare and valuable. Color is graded on a letter scale from D to Z, with D representing a colorless diamond.
The color of a diamond depends on the quantity and nature of trace elements within the stone and how these elements originated. Most commercially available diamonds range in color from colorless to pale yellowish. There are other colors of diamonds known as "fancy diamonds" which can be pink, blue, green, yellow, brown and even black.
Diamonds that are graded in the D-F range are the rarest and consequently most valuable. In reality, diamonds in the G-K range have such a small amount of color that the untrained eye can not see it and as such offer a great value.
Fancy color diamonds do not follow the grading conventions above. Learn more about Fancy yellow diamond grading.