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The 4Cs
There was no systematic approach to the grading of diamonds until the 1940s, when the Gemological Institute of America developed its International Grading System and the guidelines popularly known as the 4Cs (carat, cut, clarity and color). While a yellow diamond will be graded for carat, cut and clarity in the ordinary way, it will undergo a different color grading system.
Color Grading
Tested for the presence of color under controlled lighting conditions with comparisons made to gems of known quality, fancy diamonds are graded yellow, pink, blue and so on, then assigned a "fancy grade" such as light, intense, vivid or deep.
Certification
A yellow diamond that has been through this process will have a certificate or report stating its color grade and whether that color is natural or synthetic in origin, as well as giving details of its clarity and weight in carats.
Having a Yellow Diamond Certified
The Gemological Institute of America offers a color grading service that costs about $140 as of 2010 depending on the carat of the stone. Note that they test only loose, unmounted natural gems.
An Uncertified Yellow Diamond
If a fancy yellow diamond lacks certification, that could mean either that it is lab-grown or that the color has been artificially induced.