The girdle is the widest edge, or circumference, of a diamond. It divides the crown from the pavilion. This is usually the point of the diamond that makes contact with the jewelry setting, so it is important that it is not too thin. When measuring a diamond in millimeters, the girdle is the part that is measured.
In texture, this part of a diamond can be faceted, unfaceted (or “bruted”), or polished. This usually depends on the size of the diamond and ideally is faceted and polished, but it is not as important as the thickness.
Girdle is considered part of the cut grading system for the GIA. Both the average girdle thickness percentage and minimum/maximum verbal descriptions are important. Girdles are generally visually assessed by a gemologist because every diamond is so different in angles above and below the girdle. The gemologist takes photos and magnifies by 10 to accurately record the girdle. It may be graded Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick, or Extremely Thick.
Thin, Medium, or Slightly Thick are the preferred grades for a good diamond. On the thin side, a diamond is at a higher risk of chipping, and when it is thick, it adds unnecessary weight to the diamond and makes it look smaller than it is. Diamond shape may affect the ideal girdle thickness, so make sure you’re working with a jeweler or diamond dealer you trust, and never purchase a diamond without its certificate from the GIA or AGS.