A diamond’s outermost edge is also known as the girdle. Its thickness is measured as a percentage of the diameter (e.g. thin = 1.0%, medium = 3.0%, thick = 4.0% etc…) or quoted as words (such as Thick or Thin). On a grading report, the finish of the girdle will also be stated and listed as one of the following: polished, faceted or bruted.
First of all, the ideal girdle thickness not be too thin such that it causes additional risks of chipping. On the other hand, the girdle thickness shouldn’t be too thick as it will result in “dead weight” and makes the diamond appear smaller.
You should also beware of diamonds with extreme girdle thickness variations (e.g. very thin – extremely thick) as they usually indicate poor cut. Besides lower optical performance, extreme variations can also cause problems during the setting process.
The finishing of the girdle may also create undesirable appearances for the diamond. For example, if a bruted girdle is sufficiently thick enough, it can cause an unsightly gray reflection to appear under the table facet. As a general guideline, I only recommend buying diamonds with polished or faceted girdle finishing.
The girdle descriptions are relative to the size of the stone (i.e. diameter). In other words, a measured thickness of 0.31mm may result in a “Very Thick” rating for a 1/2-carat round diamond. However, a measured thickness of 0.31mm will result in a “Thin-Medium” rating on a 5-carat diamond.
If the girdle thickness is extremely thin, the facets junctions of the crown and pavilion facets meet together to form a knife edge. From a mechanical viewpoint, the diamond’s crystalline structure is weakest at knife edges and makes them very susceptible to damage.
In the photograph above, you can see a knife-edge girdle that has nicks and chips.
Girdle thickness is classified into categories ranging from extremely thin to extremely thick. Diamonds with girdles that lie in the outer regions of extremely thick and extremely thin have cut related issues in them. This is one of the reasons why such stones are usually less valuable and sell for a lot less.
The gemological labs assign a description based on the thickest and thinnest portions of the girdle. Also, it’s common to see variations in girdle thickness across a diamond instead of a nice uniform thickness throughout the diameter.
Slightly Thick – Very Thick
Very Thin – Thick
The above examples present 2 round brilliant cuts with different problems. The diamond on the left has weight retained at the girdle area and results in a smaller looking diamond from the face up view. The one on the right poses a serious durability problem due to the very thin section of the girdle.
Also, just by looking at these profile views, you can actually deduce that these diamonds have problems with their overall symmetry.
Ex. Thick – Measurement: 6.00 * 6.06 * 4.14 mm
Medium – Measurement: 6.49 * 6.47 * 3.94 mm
An extremely thick girdle causes excessive weight retention which is hidden from sight after mounting. This is also one of the reasons why 2 stones with similar carat weight can face up completely different and have stark physical dimensions from each other.
Did you know that GIA’s grading system will actually penalize and downgrade the diamond’s cut rating because of the girdle thickness it has? Using the 2 examples above for reference, the cut grade of the diamond on the left was graded as Fair while the one on the right received an excellent rating.
Note: This table is only applicable for round diamonds. Source: Culet And Girdle Assessment, GIA
The girdle thickness increases near both tips of this marquise shaped diamond.
It is acceptable for diamond shapes such as the heart, pear or marquise to have thicker girdle thicknesses at the locations of the pointed edges. In this case, the extra thickness provides additional strength and acts as protection against chipping.