There’s a lot to consider when picking out a diamond. Most people start with the 4 C’s (cut, color, carat, and clarity), but that just scratches the surface. One important– yet often overlooked– factor to consider is the diamond’s girdle.
What is Girdle?
The girdle is the part of the diamond where the crown meets the pavilion. It’s also the outer edge of a diamond and its widest part. When measuring a loose diamond, the measurement is taken from the girdle.
The girdle also acts as the diamond’s setting edge; after a diamond has been set it is only the crown, the area above the girdle, that can be seen.
How the Girdle is Measured
Girdle thickness can be measured in millimeters, then expressed as a percentage of the diamond’s diameter. However, when it comes to thickness ratings, the girdle’s grade is based on visual observation.
The girdle thickness varies widely between diamonds, and it is graded on a scale ranging from extremely thin to extremely thick.
There is almost always a range in girdle thickness; that is, the thickness fluctuates around the stone. For this reason, it is not unusual to see a diamond with a girdle thickness of Very Thin to Medium, or Medium to Extremely Thick.
Ideal Girdle Thickness
Ideally, girdle thickness will fall between Very Thin and Thick.
A stone with an Extremely Thin girdle is prone to chipping, making it a high-risk purchase. Very Thin diamonds, while considered a very good gemstone proportion, also carry this risk. This means extra care must be taken when setting the stone, and the girdle should not be exposed.
Very Thick and Extremely Thick girdles also have disadvantages. The increased thickness of the girdle may keep the stone from being as bright as it would otherwise be, and could make the stone appear smaller.
Because girdle thickness is often presented in a range, it can be difficult to decide if you are getting a good cut. What happens, for example, if the stone is Extremely Thin to Slightly Thick? Even though Slightly Thick is considered ideal, there could be some apprehension due to the Extremely Thin rating. When this happens, it needs to be analyzed on a case-by-case basis. It may be only a very small portion of the stone that is Extremely Thin, or it may be the majority of the stone. Without examining the diamond more closely, it is impossible to tell.
Types of Finishes
It is only the thickness of the girdle that is graded by the Gemological Institute of America (GIA), but the girdle’s finish is still worth mentioning. There are three different types of finishes: bruted (or standard), faceted and polished.
While it is no longer common, in the past bruting was the standard way to finish a girdle. During the process, two diamonds are ground together to create a finish similar to the ground glass. This leaves the stone with a dull or milky look.
These days, if a diamond is bruted it is usually given a polished finish. This creates a smooth, clear look.
The most common type of girdle finish is now faceted. This means a series of tiny facets– nearly 100– are cut into the girdle’s edge. The facets are then polished, creating a brilliant diamond.
This article was contributed by Frank Fisher.