Evaluating Diamond Cut

To properly judge the cut of a diamond, you must know the Table Diameter (%), the Crown Angle (in degrees), the Pavilion Depth (%), the Girdle Thickness (%), and the Culet Size… This sort of information is commonly found for diamonds which have a certification (commonly from GIA, AGS, EGL or AGL).

For example, the American Gemological Society (AGS) gives these proportions for “ideal” round brilliant cut diamonds:

Table Diameter: 52.4 – 57.5%
Crown Angle: 33.7 – 35.8› Degrees
Pavilion Depth: 42.2 – 43.8%
Girdle Thickness: Thin, Medium, Slightly Thick
Culet Size: None, Pointed, Very Small, Small, or Medium

You’ll often find these proportions, or ones close to them, called the “American Ideal” cut. Note that no Total Depth measurement is specified in the AGS Scale… Just the same, gemologists prefer that the total depth of a round brilliant cut diamond be somewhere between 60.0% and 62.5% (that’s % of the diameter of the stone, by the way). Although the AGS specifications allow “ideal” stones with total depths as deep as 63.5% gemologists recommend that you avoid them if you’re really seeking an “ideal” stone.

Please be aware you cannot determine the cut grade of a diamond without knowing all of the proportions. One can often find diamonds being called “ideal cut” only because the table diameter and total depth appear to be in the ideal range. But, you need all the colors of the palette to complete the picture. While a diamond that has a 55% Table and a 60% Total Depth is likely to be better than many in terms of brilliance, those numbers alone don’t make it an “Ideal Cut.” If the diamond is not priced with an “ideal cut” premium, it may well be a great deal on a very attractive stone! Just don’t let anyone tell you it’s “ideal” based on two measures alone – and then try to charge you for it!

This is an important consideration for those of you who see GIA graded diamonds. GIA certifications are used frequently and are highly respected by gemologists and appraisers. It’s also the best report to have when insuring a diamond: Nobody argues with a GIA report! But upon looking at a GIA report, you may find yourself wondering where all the specifications are! No crown angles are listed on a GIA report! Not to worry. Although GIA does not list crown angle on their reports, they DO indicate in their “notes” section if the crown angles are outside appropriate boundaries.

There is more than one “ideal” cut, though. Marcel Tolkowsky, a physicist and member of a Belgian diamond cutting family, published Diamond Design, the first scientific analysis of diamond proportions in 1919. His work was based on then- modern theories of light behavior and his opinion of what proportions resulted in the best possible balance of brilliance and dispersion of light.

Tolkowsky’s calculations for the ideal angles and proportions for a round brilliant cut diamond are as follows:

Table Width 53%
Total Depth 59.3%
Crown Angle 16.2% or 34› Degrees
Pavilion Depth 43.1% or 40›
Girdle Edge Thin

Notice that Tolkowsky specified that the girdle edge of the diamond be Thin. In light theory, that makes for a great deal of brilliance, but a girdle edge can be chipped when extremely thin. For this reason, most diamonds cut to Tolkowsky standards will be at least medium-thin on the girdle. This is not a major concern, and other standards now call for this. Things HAVE changed just a bit since 1919! While Ideal cut stones are, of course, the ideal, that doesn’t mean a non-ideal stone is terrible. Often a well-cut stone that doesn’t make the “ideal” cutoff is still very attractive with good brilliance and fire, and good value – since it should be priced accordingly lower. For your reference, we provide below the American Gemological Society specs for other well-cut-but-not-ideal diamonds.

AGS-1 Excellent Proportions

Table Width % 51.4 – 52.3% -or- 57.6 – 59.5%
Crown Angle Degrees 32.7› – 33.6› -or- 35.9› – 36.3›
Pavilion Depth % 43.9 – 44.3%
Girdle Thickness Very Thin
Culet Size Slightly Large

AGS-2 Very Good Proportions

Table Width % 59.6 – 61.5%
Crown Angle Degrees 32.2› – 32.6› -or- 36.4› – 36.8›
Pavilion Depth % 41.7 – 42.1% -or- 44.4 – 44.8%
Girdle Thickness Very Thin to Slightly Thick
Culet Size Slightly Large or Smaller

AGS-3 Good Proportions

Table Width % 50.4 – 51.3% -or- 61.6 – 63.5%
Crown Angle – Degrees 31.7› – 32.1› -or- 36.9› – 37.3›
Pavilion Depth % 41.7 – 42.1% -or- 44.4 – 44.8%
Girdle Thickness Thick
Culet Size Large

As to diamonds without a certification: Unless you’re comparing diamonds graded by a laboratory, it is unlikely you can determine the actual proportions of a diamond. Most diamonds sold are NOT graded. There’s little point in doing so for smaller diamonds. In these cases, armed with knowledge, you can simply select them based on how brilliant they look and price them accordingly.

Bottom line, folks: you’re buying a diamond because you want it to LOOK GOOD. If it does, you’re getting what you want. If it doesn’t, you’re not. If a diamond looks dull and lifeless, an informed diamond buyer knows that it’s poorly cut. Let those stones go to the inexperienced buyer who only considers size, clarity, and color when shopping for a diamond.