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Gemstone cutters have always known that the faceting on a gemstone is what gives it ‘life’. Even before there were grading certificates, traders would factor in poor cut proportions into the price of diamonds. It’s now universally accepted that cut is the most important factor out of the 4Cs of diamonds so you better pay attention.

This tutorial is designed to get you familiarized with some of the cut terminology that you’ll come across in the early stages of your diamond search.

Cut Proportions

If you want to become a diamond consumer, you’re going to need to know about the different facets that make up a diamond. A modern round brilliant has 57 facets (58 including the culet) and they are the following:

  • Table – Represented by the “Table Percentage” and measured as a % of the diameter.
  • Crown – Represented by the “Crown Height” measured as a % of the diameter and the “Crown Angle” measured as the angle the crown facet makes with the girdle. The crown is also known as the kite and bezel facet.
  • Pavilion – Represented by the “Pavilion Depth” measured as a % of the diameter and the “Pavilion Angle” measured as the angle the pavilion makes with the girdle.
  • Girdle – Represented by the “Girdle Thickness”, which is a description of its thickness and a measurement as a % of the diameter.
  • Culet – Represented as a description of its size.
  • Lower Girdle – Represented by the “Lower Girdle Length” measured as a % of how far it extends toward the culet.
  • Upper Girdle – Not represented on lab reports but is usually referred to by the “Upper Girdle Angle” measured as the angle the upper girdle makes with the girdle.
  • Star Facet – Represented by the “Star Length” measured as a % of the distance between the table and the girdle edge.

Very Good Cut is Not Very Good

GIA cut grade is divided into 5 categories (Excellent, Very Good, Good, Fair, and Poor). AGS adopts a different system (0 – 10). The AGS cut grades can be categorised into six categories (Ideal = 0, Excellent = 1, Very Good = 2, Good = 3-4, Fair = 5-7, Poor = 8-10). The most important thing to note here is that the grading systems of AGS and GIA are different and a GIA Excellent does not equal to an AGS 0. In general, the AGS system is seen as stricter in terms of cut and a GIA Excellent cut can be anywhere between an AGS 0-2.

Industry experts suggest that you should stay within the excellent and very good cut grades, but what you’ll find is that most diamonds sent for grading will fall within these two grades. In fact, I would never recommend a diamond that doesn’t have a very good or excellent cut.

If you value cut, you should definitely stay within the GIA Excellent/AGS0 cut grade. However, remember that a GIA Excellent/AGS0 cut grade does not necessarily make a diamond the absolute best cut nor even something very close. The reality is that a very good cut grade is usually not really very good at all.

Symmetry and Polish

Related to cut is the finish quality of the diamond. A diamond’s finish is graded on symmetry and polish and both refer to the level of craftsmanship in producing the diamond. Both are graded on the same scale as cut. You may have heard of the terms “GIA Triple Excellent-cut” and “AGS Triple Ideal-cut”, this refers to Excellent or Ideal grades for cut, symmetry, and polish. The AGS Triple Ideal-cut is commonly misrepresented as an “AGS Triple Zero” (AGS000), which strictly speaking, only refers to a diamond that receives a 0-grades in cut, color, and clarity..

Often, when reselling a diamond, a diamond will need to be re-polished and a new certificate issued. There is a risk to getting a diamond re-graded because there is no guarantee that the same grade will be rewarded. Diamonds that have older certificates (more than 1 year) should be checked out by a professional appraiser.

Re-polishing is how diamonds are recycled and even a diamond with a new certificate does not mean that it has not been pre-owned and you will probably never know. It is only certificates from before 1 June 2005 for the AGS and 1 January 2006 for the GIA that you have to be concerned about as these certificates are outdated. 

The Ideal-cut

The ideal-cut is a misnomer because the term ideal gives the impression that it is perfect. The ideal-cut really just refers to a diamond that would receive the top grade of 0 in cut from the AGS. However, today the term is frequently misused to refer to GIA triple excellent cut diamonds as well.

Shallow and Steep-deep Diamonds

Shallow and steep-deep diamonds typically refer to poor combinations of crown and pavilion angles that lead to either a loss in fire and sparkle in the case of shallow diamonds, or a loss in brightness in the case of steep-deep diamonds.

The 60/60 Diamond

A 60/60 diamond refers to a diamond with a 60% table and a 60% total depth. This term was adopted because traditionally experts believed you could filter out diamonds simply using these two proportions. These diamonds are no longer considered ideal cut, although some people still seek them out for their brightness and spread, as they are commonly paired with shallow crown angles. The term ‘spread’ refers to how big a diamond looks for its carat weight.

Tolkowsky Ideal-cut

The Tolkowsky Ideal-cut (TIC) is named after Marcel Tolkowsky, a mathematician who famously published the proportions making up the perfect diamond in his thesis titled “Diamond Design, A Study of the Reflection and Refraction of Light in a Diamond”(1919).

Tolkowsky published a unique set of proportions:

  • Table Percentage = 53%
  • Pavilion Depth = 43.1%
  • Pavilion Angle = 40.75 degrees
  • Crown Height = 16.2%
  • Crown Angle = 34.5 degrees

The TIC evolved to become the American Ideal-cut, which changed the ideal crown height to 14.6%. In 1969, the Scandinavian Standard-cut was published which was based on the American Ideal-cut with the ideal table percentage changed to 57.5%. The Scandinavian Standard-cut is in my opinion the set of proportions that produces the perfect balance between light performance and spread.

Today, the term TIC is used to refer to the range of proportions that surround the exact Tolkowsky numbers. This is because it has been found that there is a range of combinations of proportions that produce what can be appropriately deemed ideal. Do not confuse the Tolkowsky Ideal-cut with the Tolkowsky brand of diamonds which are not all TIC diamonds.

Super-ideal Cut

The super-ideal cut is a combination of the TIC range of proportions and perfect optical symmetry representing the pinnacle of diamond cutting. A super-ideal diamond will display perfect hearts and arrows and have the best light performance. Super-ideal cut diamonds do not happen by luck, they are carefully planned from the rough diamond stage and not all rough diamonds can be cut into super-ideal cut diamonds.

This means that if you’re looking to purchase a super-ideal cut diamond, you need to look for vendors who specifically carry a line of super-ideal cut diamonds and be prepared to pay 10-20% more for a super-ideal cut. Bear in mind that not all branded diamonds or those that are marketed as hearts and arrows diamonds are super-ideal cut.

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