We all know that a belt can save or ruin any outfit. The same applies to the diamond girdle. Being a small part of a polished diamond, girdle plays an important role in measuring proportions of a stone and evaluating its cut quality.
What is a Diamond Girdle?
The girdle is a narrow portion of a diamond separating the top (crown) and bottom (pavilion) parts of the stone. Being the widest part of any polished diamond, girdle functions as the setting edge and plays an important role when measuring diamond proportions.
To determine how well a diamond is cut the other parts of the stone are compared with the girdle in terms of size. For example, diamond total depth, crown height, pavilion depth and culet size are expressed as a percentage of the girdle.
Total Depth % = Total Depth (mm) / Average Girdle Diameter x 100
Crown Height % = Crown Height (mm) / Average Girdle Diameter x 100
Pavilion Depth % = Pavilion Depth (mm) / Average Girdle Diameter x 100
Culet Size % = Culet Size (mm) / Average Girdle Diameter x 100
The girdle is also an ideal location for laser inscriptions that help identify a particular stone among others without affecting its appearance.
Diamond Girdle Types
There are three different types of diamond girdle: rough (also referred to as unpolished or bruted), polished and faceted.
In old times, diamond girdles were left unpolished in their original condition after the bruting process where two diamonds are rotated against each other to give them a round outline. That is why rough girdles are also referred to as “bruted”. Such girdles have a frosted appearance and tend to absorb dirt and oils requiring more frequent cleaning as a result.
If too much pressure is applied during the bruting stage, a diamond girdle can get lots of micro feathers that look like tiny hairs. Such rough girdles are referred to as “bearded”.
Nowadays, with the advent of diamond cutting techniques, polished and faceted girdles are seen more frequently. Rectilinear (princess) and step-cut diamonds (emerald and Asscher) almost always have flat polished girdles, while roundish diamonds (round, oval, pear, heart) usually feature faceted girdles.
Diamond Girdle Thickness
Diamond girdle thickness is measured by highly accurate optical devices in labs. It is expressed as a percentage of the average diameter using the following formula:
Girdle Thickness % = Girdle Thickness (mm) / Average Diameter x 100
Girdle thickness is evaluated based on the range from the thinnest to thickest valley positions. Valley positions are the thin areas located where the upper and lower half facets come closest.
Verbally girdle thickness is described using an 8-grade scale: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick and Extremely Thick.
If the thinnest and thickest points fall into the same category, a single rating is given. However, in most cases, girdles are rated as a range to cover the variance between the thinnest and thickest points, for example Very Thin to Thick.
It is important to understand that diamond girdle ratings are relative to the size of the stone. For example, 0.50ct diamond with 0.31mm girdle may be assigned a “Very Thick” grade while a 5.00ct diamond with the same girdle thickness will result in “Thin – Medium” range.
Is the Girdle Thickness Important?
The thickness of the girdle is one of the most important characteristics.
An Extremely Thin girdle, also referred to as “knife-edge”, may easily break or chip when setting the diamond in a mounting or while being worn. On the contrary, if the diamond’s girdle is Extremely Thick, its additional size will have come at the expense of other parts of the stone and proportions won’t be optimal.
The ideal girdle thickness should be either Thin or Medium. Slightly Thick is also very close to ideal proportions. Such grades as Very Thin and Very Thick are still good gemstone proportions; however, care should be taken while setting the stone.
Remember to avoid girdles that are Extremely Thin, Very Thick or Extremely Thick as it indicates a disproportional cut.
Irregular girdles, meaning some portions of it are thinner and others are much thicker, are also problematic. In this case, the facets immediately above and below the girdle are not well aligned and are disproportional.