Gem-quality diamonds occur in many hues. When light enters a diamond, it breaks into its spectral elements - brilliant reds, blues, violets - coloring the diamond.
The most valuable diamonds are colorless. The absence of color allows the most natural light to pass through the diamond. A truly colorless diamond is extremely rare. Most diamonds possess varying degrees of yellow or brown, and small, subtle differences in color can make a substantial difference in value.
The American Gem Society grades the diamond color on a scale from 0 (zero) to 10, zero being colorless.
The Gemeological Institute of America uses letters from D to Z for its color grading scale, D being colorless.