The second C, Color, evaluates the degree to which a diamond is colorless. The color of a diamond is graded on a scale from D (colorless) to Z (light yellow or brown). The closer to colorless the diamond is, the rarer and more valuable it becomes. The difference between a D-color diamond and a G-color diamond can be subtle, but it's significant in terms of price. Colorless diamonds are highly prized for their rarity and brilliance.