The thin perimeter of a diamond that divides the crown above from the pavilion below is called girdle. Examining a polished diamond from a profile view girdle is the widest part i.e. the part of the stone that is in physical contact with the setting itself. While measuring loose diamonds it is ensured that the measurement is done by the girdle in order to obtain length and width in millimetres.
A diamond with a brilliant cut usually has a faceted girdle and not a round girdle which makes the gem more transparent. It is important to note that a diamond girdle can be rough, polished or faceted.
For the grader to determine a diamonds thickest and thinnest points it is measured at various points along the girdle while getting certified and if both the points come into similar category, the rating given is also single. The girdle rating is done between very Extremely thin to extremely thick which is as follows:
Might chip or break; the setting of stone should be carefully done
Proportion of the gemstone is very good; the setting of stone should be carefully done
This is the ideal proportion
Slightly thick, Thick
This range falls between ideal and excellent in terms of proportion
Proportion is good
It might make the diamond appear a tad bit smaller because of the depth taken by the girdle.