Color refers to the natural body color of a diamond and not to the reflection of spectral colors that flash when a diamond moves. Most diamonds naturally exhibit slight hints of yellow, brown, or gray. This color is caused by natural trace elements of nitrogen that were present when the diamond formed under the Earth's crust. Diamonds with less color allow more light to pass, releasing more brilliance and fire. A diamond acts as a prism by dividing light into a spectrum of colors and reflecting this light as colorful flashes called dispersion or fire. Similar to looking through colored glass, color in a diamond will diminish the spectrum of color, or fire, emitted by acting as a filter.
Diamonds are valued by how closely they approach colorlessness – the less color, the higher their value. The only exceptions are the fancy-color diamonds, often blue, brown, pink or yellow, that lie outside of this range. Fancy-color diamonds have a natural color that is deep, distinct, and opulent. Their color is more saturated than a Z color diamond and they are graded based on the hue, tone and saturation of their color -- the more intense the color, the higher the rarity factor.
GIA's color-grading scale for diamonds is the industry standard. The scale begins with the letter D, representing colorless, and continues with increasing presence of color to the letter Z, or near-colorless. Why start with D? Before GIA developed the D-Z Color Grading Scale, a variety of other systems were loosely applied. These included letters of the alphabet (A, B and C, with multiple A's for the best stones), Arabic (0, 1, 2, 3) and Roman (I, II, III) numerals, and descriptions such as "gem blue" or "blue white." The result of all these grading systems was inconsistency and inaccuracy. Because the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D—a letter grade normally not associated with top quality.