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
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.
Looking from the macro level the girdle does not need a significant consideration since thinness or the thickness of a girdle does not have much impact on the look of the diamond that is well cut. However, while assessing the symmetry of the diamond the variation between the thick and thin points is taken into consideration because a significant variation can negatively affect the look of the diamond by making it appear more disproportionate. Buyers looking for diamonds with ideal cuts should look for a diamond that has a girdle rating between thin and thick. Diamond that has girdle rating off medium is considered to be perfectly proportioned with expert cut.The girdle effect
A girdle that is rated as extremely thin has a slight risk of chipping along the girdle edge while the stone is being state or the jewelry is worn. To protect it properly the diamond has to be well secured within the setting which will reduce the possibility of contact with rough or hard surfaces. If a diamond is cut ideally then the diamond with an extremely thick girdle will have great brilliance and fire.
Diamonds that have girdle ratings of extremely thick or extremely thin are available at cheaper rates than diamonds that have ideal girdle ratings which offer great value to the buyer.