What is the Diamond Girdle and What Does it Matter

The diamond girdle is a small but very important part of the anatomy of a polished diamond. It has practical as well as aesthetic properties, and is a significant factor taken into consideration in laboratory grading.

The girdle is the small vertical plane on the perimeter of the diamond that helps define its shape and acts as a border between crown and pavilion.

The girdle can be polished, un-polished, or faceted. In the early years most diamond girdles were left unpolished in their original condition after the ‘bruting’ stage where two diamonds are rotated against one another giving them a round outline and establishing the girdle plane. Unpolished girdles are also referred to as ‘bruted’ and have a frosted appearance. They have a tendency to absorb dirt and oils making the stone darken somewhat over time and requiring professional cleaning.

If too much pressure is applied during the bruting process, the girdle can acquire an abundance of micro feathers extending into the diamond that look like tiny hairs. This unfortunate characteristic is referred to as a ‘bearded” girdle.

With the advent of better equipment enabling cutters to be more precise, faceted girdles are seen more frequently today, particularly on brilliant style facet designs such as rounds and ovals. Rectilinear cuts such as Princess and Emerald cut almost always have flat polished girdles.

The symmetry grade issued by the labs is an evaluation of how consistently the facets meet at the proper places around the diamond. This is known as “meet point symmetry”. (This differs from ‘optical symmetry’, the precision of the placement of corresponding facets in three dimensional space). The girdle is examined carefully in the grading process for uniformity in the alignment of crown and pavilion facets as part of establishing the symmetry grade.

The girdle also serves to function as a safe barrier against damage to the edge of the diamond, both during setting and during daily wear. Very thin girdles can make the stone more vulnerable to abrasion and chipping.

The girdle also provides an ideal location on the diamond to place a discrete laser inscription that helps identify the diamond without affecting its appearance. Since the technology became available some twenty years ago, inscriptions have proven very popular with both merchants and consumers alike. Merchants can better control their inventory by avoiding accidental mix-ups. Consumers have an independent way to quickly verify their diamond upon purchase and on subsequent trips to the jeweler for repairs.

Girdle thickness (along with all other aspects) is measured at the labs by highly accurate non-contact scanners such as Sarine. Girdle thickness is expressed mathematically as a percentage of average diameter. An extremely thin girdle could be a percentage as low as 0.0% and extremely thick girdle could be as high as 10.0%.

The range of descriptors is: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very thick, and Extremely Thick. Lab reports will typically provide a range of thickness around the stone, e.g. Medium to Slightly Thick. If it is extremely consistent all the way around it will have one designation such as Medium.

As stated above, the girdle is the junction of the crown facets and the pavilion facets. This gives rise to a slightly scalloped effect caused by different sets of facets meeting at different points. Therefore the thickness of the girdle undulates somewhat in a series of hills and valleys. The GIA and AGS report girdle thickness somewhat differently. AGS measures the thickest and thinnest areas anywhere around the girdle. GIA report the thinnest and thickest measurements at the valleys, so the report tends to show less variation and a smaller measure.

The girdle is a small but important feature of a polished diamond. Cut quality and durability are both impacted by the design and execution of the girdle. Both GIA and AGS cut grading systems analyze the girdle and any significant issues with the girdle will result in a grading deduction. Careful examination of the girdle can reveal things such as symmetry faults, repairs resulting in extra facets, and potential swindling techniques such as painting and digging. Lastly, the girdle plane provides the perfect location to put a laser inscription for quick identification without detracting from the visual appearance of the diamond.