Articles: The AGA Charts versus the Holloway Cut Advisor
by David Atlas
When I made these charts there was no HCA. I referred to the parameters I found in actual finely cut diamonds to give the range of what appeared to create the very "best" subjective ranges. Some of the possible permutations cannot exist and maybe that is what you are finding. You are near the shallow limit in total depth so the only variable is girdle thickness which may calculate to below zero and then simply not be a possible combination.
Besides, the HCA is a way to make a judgment on a stone you cannot see. The AGA charts are a way to help make judgments on actual stones that you have in hand to view.
Many times the HCA and AGA charts agree. However, the theoretical stone must be possible. The HCA can predict when you have chosen an impossible combination of parameters. No problem.
The best way to find a diamond is to look for actual stones. Trying to disprove or discredit the AGA charts is okay with me, but besides being thoroughly reviewed and used by the trade for more than 15 years now, you will find that actual diamonds recognized by the vast majority of the diamond business that rate a fine cut with high brilliancy are within the parameters set forth. The HCA will frequently do the same and also give you guidance on many diamonds outside the region of the present recognized "ideal" parameters, but also ones which have excellent light return and great looks.
While somewhat compatible, the HCA and the AGA charts are not joined in some mystical way. They are distinctly different approaches from different ideas of what constitutes fine diamonds. The AGA is conservative based on a traditional approach. The HCA is a combination of traditional and new thinking. Both seem to be good helpers in finding pretty diamonds.
A comment from Garry Holloway:
It is no coincidence that Dave and I met up and became friends. More than a decade before we met I came across Dave’s fancy cut charts and immediately began using them (with permission) in diamond cut lectures at the Gemmological Association of Australia and to Registered Valuers (Appraisers in USA talk). A few years before (1984) I had taken the diamond course and did well enough to be invited to run it. I was fortunate enough to acquire a Firescope and did a small paper on it as part of the course requirement (followed by a presentation at the national congress).
The Firescope changed the way I examined diamonds and led to the development of a portable hand held version which we used to buy the diamonds for my business; this is of course how the Ideal-Scope was born. So when we released the Ideal-Scope and Dave Atlas ordered 500, you might imagine that I was very proud to have begun a formal association.
Using the Ideal-Scope as we do, examining often a hundred stones for every one we buy, I noticed that a much wider range of proportions produced beautiful looking diamonds than those assumed by the trade at large. Thus it was that I became the first person (I believe) to discover an inverse relationship between crown and pavilion angles. Diamonds with steeper crowns can retain their beauty if they have a shallower pavilion, and vice a versa.
I was way out in the actual mathematical equations in my original estimations, but that all changed when I began communicating via the Internet with my new friends from Moscow State Uni. So with thier amazing software package called DiamCalc I modelled thousands of virtual diamonds with digital perfect proportions and checked every diamond with known proportions with our Ideal-Scopes and created Holloway Cut Adviser (HCA).
Lots of ‘Cut Nuts’ have now noted that applying tolerances to Tolkowsky proportions the way large institutions do brakes the rules on this inverse relationship. The combination of a 35.8° Crown angle and a 41.2° pavilion used by AGS as the upper limit of their best grade performs like equates to a diamond with Tolkowsky’s pavilion angle and a crown angle of 38°, which any gemologist will tell you is a real dog. (Actually I prefer the 38° crown than the steep deep the AGS 0).
Anyway the point is that Dave has refused to be pressured by cutters to give a looser range of proportions (it’s a big deal, those diamonds weigh 2-3% more). So the AGA chart has always ensured consumers get a diamond with the very best performance. And Dave rightly points out that some the proportions with low HCA scores are not favoured by those who are used to a certain type of look, nor will they have an easy resale valuable, not that is until the GIA releases a cut grading system. Then I believe the savvy and brave souls who take my advice can get some good buying.
Happy Diamond Hunting
HCA scores and comments for the worst performing proportions in the top grades of a number of grading systems in use around the world today.