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Ideal Cut Proportions

Some content on this page are contributed by Good Old Gold, with thanks.

What is the Ideal Cut?

Quite simply, the ideal cut ensures that a diamond's light return is optimised. That would mean that the diamond would have excellent brightness, contrast, fire and scintillation by anyone's standards.

This is a site to help you find your proposal ring. So, I do not think that you are going to settle for a diamond of less than ideal proportions for your soon-to-be fiancé. Hence I am not going to waste time on anything less than ideal.

Fortunately for you, if you are selecting a diamond today, you can rely on both the Gemological Institute of America (GIA) and American Gemology Society (AGS) diamond reports for their cut assessment. That is because both these highly respectable gemological laboratories launched their new diamond cut grading in January 2006. So, any reports after that date will contain a highly reliable cut grading (with a few notable flaws which I will point out later).

Before 2006, even the most respected experts in the field cannot agree on what exactly is the ideal cut. For that reason, I am going to share with you several past views on what would be considered the ideal proportions. Hopefully, at the end of this section, you would be able to appreciate what AGS and GIA have done.

I must emphasis that it is possible to find a diamond that is not of ideal proportions, but offer excellent light return under specific lighting conditions (like in the jewellery shop). Although these offer good value, I am not going to discuss them here because I am sure you would want your fiancé to wear her proposal ring anywhere she pleases (under any lighting condition). Only diamonds cut to ideal proportions perform under most lighting conditions.

A little History: The Tolkowsky Ideal Cut Proportion

In 1919, Marcel Tolkowsky published Diamond Design. He systematically analyzed the optics of a diamond. He estimated the best proportion for cutting round brilliant diamonds. With minor changes, today's standards for "ideal cut" diamonds are based on Tolkowsky's book. Tolkowsky suggested that the following diamond proportion will produce a diamond with the best brilliance, fire and scintillation:

However, diamonds need not follow the above proportions to achieve excellent light return. As explained in the article on the table, bezel facets and pavilion mains, we can vary these measurements and still obtain good light return. How exactly should we set the standards of ideal became a challenge because taste plays a part in deciding what qualifies and ideal.

Why is an Ideal Cut diamond so expensive?

Actually, a ideal cut diamond offers more value for money. As I have said, an ideal cut diamond appears bigger. Many people have commented that my wife's diamond looks very big. It is, in fact, only 0.62ct. Not to mention that if a diamond is badly cut, it is in fact an ugly looking thing.

To cut an average cut diamond to ideal cut proportions, the diamond will lose about an additional 20% of weight. We have not even considered the labour of the master cutter required to cut the diamond to ideal proportions. So, if you have bought a poorly cut 1ct diamond, you should have paid the price of a 0.7+ct ideal cut diamond. If you have paid anything more than that, then you have been ripped off.

The difference in price which can be attributed to cut proportions alone is easily 40% of the diamond's value.

The Old AGS Ideal Cut Proportion

A diamond cut to Tolkowsky's proportion would be stunning. However, modern research shows that it is not necessary to adhere strictly to Tolkowsky's calculations to achieve maximum light return. It is possible to deviate from Tolkowsky's proportions and still achieve excellent light return. Before 2006, the AGS would grade a diamond's proportions as ideal if it embraces the following tolerances in proportions:

AGS Ideal Cut Proportions (Grade 0)
Table 53-57.5%
Crown Angles 33.7° - 35.8°
Pavilion Angles 40.5° - 41.5°
Pavilion Depth 42.2 - 43.8%
Culet Size None to Medium
Girdle Thin, Medium or Slightly Thick


Did the Old AGS Triple Ideal for Cut gaurantee performance?

The answer is quite simply no. The problem lies with AGS' tolerances in proportions, which is far too wide.

Frequently, diamond cutters cut to maximise weight (since diamond is sold by carat weight) to those ignorant about cut quality. Of course, the maximum carat weight is achieved by raising the crown angle and the pavilion angle to the maximum allowed. This diamond will have a leaky ring around the center (which some call the 'ring of death'). Light return of such a diamond will be mediocre at best.

This is an example of a mediocre performing AGS Triple Ideal cut diamond:

1.238ct G VS2

In addition, many of these Triple Ideal diamonds suffer from girdle digging and painting because the cut criteria simply did not weed out these problems. (I have a colleague wearing a diamond with dug girdles and I feel a little bad for her.)

I am sure you would not be too pleased if you should have to pay the premium for and Ideal Cut diamond and settle for a mediocre performer.

Before 2006, some other researchers tightened the AGS tolerances, hoping to ensure performance. However, this was not acceptable because they would have excluded many diamonds which had absolutely stunning light return. To be ideal, all 57 facets of the round brilliant must complement each other.

Fortunately for consumers, the GIA and AGS has since come up with highly innovative cut grading systems that solved most of the old problems.

The Current GIA and AGS Ideals

Within the course of June 2005 and January 2006 we have seen the release of 2 cut grading systems by both GIA and AGS laboratories.  Both laboratories are considered the more conservative of those currently out there. HRD is comparable for their grades on other factors, but it cannot compare to GIA's and AGS' level of sophistication and reliability when it comes to cut grading.

This is really good news for consumers. Before AGS and GIA released their new cut grading system, consumers were frequently misled into buying diamonds that were cut to less than ideal standards. The old AGS Triple 0 cut grading was the most misleading of them all. However, that is all in the past.

So before we begin a more in depth look at all the things that go into determing their cut grades it is vital to understand the 'tools' each lab uses to determine their top grades.  An understanding of the tools used will better help us understand why the grades turn out as such. We will focus on what these cut gradings tell us, and more importantly, what they do not tell us. These are all essential knowledge if you want to pick out the ideal diamond for the woman of your dreams!

We shall examine each of these cut grading systems over the next few pages.






Next: The GIA Ideal...