Diamond Girdle Thickness And Why It Is Important

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.

profile proportions

Want a fast track to the ideal diamond proportions to look out for when shopping for an engagement ring? Click here to view a chart showing the best proportions for buying well cut round diamonds.

Why Is the Diamond’s Girdle Thickness Important?

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.


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.

knife edged girdle that is chipped

In the photograph above, you can see a knife-edge girdle that has nicks and chips.

Diamond Girdle Size Chart

diamond girdle thickness chart

Here is a List of Acronyms Commonly Used

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.

  • Extremely Thin – ETN, ETH, XTN, EXN
  • Very Thin – VTN, VT, VETN
  • Thin – T, TN, TH
  • Medium – M, ME, MD, MED
  • Slightly Thick – STK, ST, SLTK, SLTH
  • Thick – T, TK, TH
  • Very Thick – VTK, VTH
  • Extremely Thick – ET, EXTK, XT, XTK
  • Faceted – F, FA, FAC
  • Smooth – S, SM

How is Girdle Thickness Described on a Grading Report?

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 to very thick

Slightly Thick – Very Thick

very thin to 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.

A Comparison of 1 Carat Diamonds With Uniform Girdle Thickness

extremely thick girdle retains weight

Ex. Thick – Measurement: 6.00 * 6.06 * 4.14 mm

uniformly medium girdle size in round diamond

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.

the gia cut grade relationship

Note: This table is only applicable for round diamonds. Source: Culet And Girdle Assessment, GIA

Situations Where a Thick Girdle May Be Preferred

acceptable variances in fancy shapes

marquise with 2 pointed corners

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.

I only recommend buying diamonds with grading certificates from GIA or AGS. Besides a reliable grading report, james Allen also provides additional information like ASET/Idealscope images and magnified videos to help you make better decisions.

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  1. Melissa-
    August 31, 2016 at 2:22 am

    I’m so sorry I can’t seem to locate my last comment/question. I also wanted to see your thoughts on this diamond? Based on the dimensions will it appear larger than actual size. Is this a fair price? $17,500

  2. Paul Gian-
    August 31, 2016 at 2:29 am

    There’s no way I can offer constructive feedback on a fancy cut oval diamond based on the grading report alone.

    Read these in full:

  3. todd-
    September 29, 2016 at 1:29 am

    What do you think about lab created diamonds at brilliantearth.com. Are they really optically, physically and chemically identical to earth mined diamonds? I was looking at
    http://www.brilliantearth.com/lab-diamonds-search/view_detail/2718170/ and
    http://www.brilliantearth.com/lab-diamonds-search/view_detail/2719148/ This second ones girdle is faceted. Would the second one be better? I spoke with my girlfriend and after reading about lab created diamonds she said that she would be okay with it.

  4. Paul Gian-
    September 29, 2016 at 4:20 am

    Read this: http://beyond4cs.com/grading/girdle-thickness/does-finishing-matter/

    By proportions alone, this would be the better diamond on paper.

  5. Mohamed Salama-
    November 2, 2016 at 4:24 pm

    Hi Paul,

    When a diamond has a girdle thickness in the GIA report as medium to slightly thick (faceted) 4.0%

    What does the % here mean?

    Does the 4% here indicate the % at the slightly thick part of the girdle?

    Why is there a single % number? Shouldn’t there be one for the medium and one for the slightly thick?

    I read this http://diamondcut.gia.edu/pdfs/booklet_finish_culet_girdle.pdf

    It talks about the % in general and how it is calculated. I don’t know what it means to have one number only.

    Can you please help?

    Thanks alot!

  6. Paul Gian-
    November 3, 2016 at 4:37 am

    Girdle thickness percentage = total depth % – (crown height % + pavilion depth %)

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