The girdle is the edge of the diamond typically held by the setting.

While the visual differences are very subtle, some diamonds will have faceted girdles and others will not. With modern diamonds, the girdle is likely to have been polished. An older, now less common cutting technique, was to leave the girdle's surface unpolished. This is called a bruted girdle. All Fonder Diamond diamonds have polished girdles and are faceted.

Girdles are measured by their width, and the GIA and AGSL record them slightly differently. The AGSL lists the width of the girdle in millimeters, while the GIA brackets the measurements into a range of categories. These categories range from extremely thin to extremely thick; you can see examples of each below.

A diamond's girdle is taken into consideration when a cut grade is being determined. This means that even if the girdle is not an ideal width (thin-slightly thick), a well-cut diamond will still produce lots of brilliant sparkle.

A thicker girdle will add more weight to a diamond, but this does not translate into a visibly larger size. If the diamond is set into a ring, this is particularly true  as the extra surface area at the girdle will not be seen. While this will have a minimal effect on a diamond's price and weight, it should still be noted.

Extremely thin girdles may be prone to chipping, and should be avoided if selecting a diamond to be set in an engagement ring - especially for fancy shaped diamonds where there are corners. For other diamond jewelry, such as earrings or necklaces that will be more protected from knocks and scrapes, these widths may be acceptable.

Some fancy diamond shapes are cut so as to have more sharp corners and are thus particularly prone to chipping. Princess cut diamonds with extremely thin girdles should be avoided, as these will increase the chances of chipping while being worn.