GIA. The Gemological Institute of America — the nonprofit research and grading lab whose certificates are the most trusted in the US diamond market. A GIA-graded diamond typically resells for 15–25% more than a comparable uncertified stone.
GIA invented the 4 Cs and the modern grading vocabulary. Its grading is conservative compared with other labs, which is why professional buyers treat a GIA report as the closest thing to ground truth.
A GIA report contains the stone's measurements, the 4 Cs, fluorescence, and any treatments. Many reports also carry a laser inscription on the stone's girdle that ties the diamond to its certificate.
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