The girdle is the thin perimeter of a diamond, dividing the crown above from the pavilion below. When viewing a diamond in its setting or from a profile view, the girdle is the widest part (or the circumference) of the polished diamond - the portion of the stone that makes contact with the setting itself. When loose diamonds are measured, they are measured by the girdle to obtain length and width in millimeters.
The girdle of a diamond can be rough, polished, or faceted. In modern gemstone preparations, brilliant cut diamonds (with triangular-shaped facets) usually have a faceted girdle rather than a perfectly round girdle; this gives the gem a higher measure of transparency. Step cut diamonds (with rows of elongated facets that act as mirrors) usually have polished girdles that are not faceted.
Girdle ratings can fall into one or more of the following categories: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick, and Extremely Thick. When a loose diamond is sent for certification, it is measured at various points along the girdle to determine its thickest and thinnest points. When both points fall into the same category, a single rating is given, such as Medium. However, the girdle is most often rated as a range, such as 'Very Thin to Thick', to accommodate the variance between the thickest and thinnest points.
Girdle size can also be expressed as a percentage relative to the diameter of the diamond. A diamond with a girdle percentage of about 0.5% to 5.0% of its diameter is considered to have a well-proportioned girdle.
Below are the possible ratings that may be applied as a description of a diamond's girdle on a diamond report:
May be prone to chipping or breaking; care should be taken when setting stone
Very Good gemstone proportion; care should be taken when setting stone
Ideal gemstone proportion
Excellent to Ideal gemstone proportion
Good gemstone proportion
May make diamond look smaller, as more depth is taken up by girdle
When purchasing a loose diamond, girdle is not a significant consideration, as a thinner or thicker girdle will not necessarily have a significant impact on the appearance of an otherwise well-cut diamond. However, the variation between the thickest and thinnest points on the girdle is considered in the assessment of a diamond's symmetry, as a significant variation can make a diamond appear disproportioned. For those seeking ideal cut diamonds, we recommend looking for a stone with a girdle rated somewhere between Thin and Thick. A perfectly proportioned diamond would have a Medium girdle rating, though ratings close to Medium are also considered to be expertly cut.
A diamond with an extremely thin girdle has a very slight risk of chipping along the girdle edge during setting or while being worn. For maximum protection, the diamond should be well-secured within the setting to minimize contact with hard or abrasive surfaces. A diamond with an extremely thick girdle will still have great brilliance and fire, as long as it is cut to otherwise ideal proportions. However, an extremely thick girdle concentrates a great deal of the diamond's weight in the middle, sometimes causing it to look smaller from the top than a diamond of a similar weight with a thinner girdle. When analyzing the impact of an extremely thick girdle on a particular diamond, it's important to evaluate the overall measurements of the stone. If the measurements are within the ideal range for the diamond's shape and carat weight, the extremely thick girdle does not have a significant impact on the perceived size.
Diamonds with an extremely thick or extremely thin girdle rating are sometimes priced lower than similar diamonds with more ideal girdle ratings, offering a great value to the consumer. If you have selected a diamond with an extremely thick or extremely thin girdle, please contact a Brilliance diamond and jewelry expert for a visual inspection before purchasing.