Although the majority of gem diamonds appear to be colorless to the untrained eye, there are subtle differences in shades of color (usually yellow or brown). This body tint to the diamond is graded on a scale ranging from D (colorless) to Z (light yellow). Each letter represents a narrow range of color rather than a single shade. The body color in a diamond results from traces of other elements mixing with carbon as the diamond crystals grew. Diamonds with no traces of body color are extremely rare.