master stones GIA's GTL will do specific reports on master stones for color comparison purposes. GIA's requirements to accept a diamond for a master stone report includes .
Master stones - A set of diamonds by which jewelers can compare and grade the color of a particular stone from D to Z.
Mikimoto, Kokichi - The man who invented and, in 1916, patented, the process for culturing pearls.
Master stones are carefully selected diamonds used by diamond grading laboratories for colour comparison, they are also distributed for use by others who need to grade diamonds accurately.
Master Stones / Master Set
A set of color comparison diamonds that defines diamond color grades in the normal (D-to-Z) range.
Near Colorless ...
Master stones: Polished diamonds, or more recently Cubic Zirconia, of known color grades that are used as comparison when grading diamonds for body color.
Probably the biggest problem today is the use of "CZ" color master stones. You must never accept an appraisal that is done using anything other than a certified diamond master set containing at least five diamonds.
To accurately grade color requires comparisons with a set of pre-graded, "master stones". The highest master stone is an E; any stone that falls on or above an E is a D. Without masters, a loose diamond can only be approximated within two grades.
Differences between color grades are extremely subtle and can only be determined accurately by comparing stones to known sample stones called "Master Stones". There is actually one set of master stones owned by GIA.
Next, the diamond's color is compared to a set of master stones. The master stone set is a row of stones laid out from D to Z, representing the standard for each color grade. Each stone represents the least amount of color in its range.
This official "original set" of CIBJO master stones consists of seven brilliants of approximately one carat each which have supreme validity for all of the nineteen countries affiliated with CIBJO.
Gem Labs (Like GIA), use Master Stones to evaluate and assign Colors to Diamonds when grading them. Viewing the Diamonds in a controlled laboratory in neutral lighting conditions, face down, is the best way to judge Color.
The Completely Illustrated Course for Developing the Techniques of Master Stonesetting: A Comprehensive Picture/Text Guide on Diamond and Colored Stone Setting. Washington, DC: Adamas, 1986.
Zeiss, Walter. Das Edelsteinfassen. 2nd ed.
Diamonds are color graded by being compared to a set of master stones. Another important and well-respected grading lab is the American Gem Society Labs., which uses a similar grading system, but a numerical scale from 0 to 10 (See figure below).
 See also: Color, Master, Diamond, Stone, Facet
  
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