Master stones are reference diamonds of known color grade. A grader sets a diamond beside them to decide its D-to-Z letter.
In more detail
Light and background change how a diamond looks, so labs don't grade color by eye in a showroom. GIA uses master diamonds in a standard viewing environment, with at least two independent graders. IGI also grades against master stones, with several gemologists.
On a grading report
Behind every D-to-Z color grade on GIA and IGI reports.
Related terms
- Color grade: The color grade is a letter from D to Z for the yellow or brown tint of a near-colorless diamond. D is colorless and Z is light yellow or brown.
- Colorless (D–F): Colorless is the top range of the D-to-Z scale: the grades D, E and F.
- Near colorless (G–J): Near colorless is the D-to-Z range from G to J.