A diamond of a carat or more should come with a gem report—a gemologist's evaluation of the stone's color by letter grade (good stones are ranked no lower than I) and clarity, ranging from "flawless" (FL) to "very slight inclusions" (either VS1 or VS2) for an acceptable diamond. The cut, carat weight and measurements are also listed. The Gemological Institute of America issues most gem reports, but a few fine jewelry firms offer their own guaranteed certificates.

The Tiffany & Co. Diamond Certificate lists a diamond's distinguishing characteristics and its linear measurements, accurate to within .02 millimeters.