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The Atlas/AGA Cut Calculator

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rbjd

Shiny_Rock
Joined
Feb 4, 2003
Messages
154
I have a few questions for Dave.

I''ve been well aware of the AGA system for awhile, but I wasn''t really paying attention to some of the cut criteria you have identified that knocks some stones down a few notches. Now that I can punch numbers into your system, a couple cut factors have shot to the top of the list for dropping diamonds out of the 1A class (round brilliant) and now I''m wondering if you can clarify.

I''ve looked at a number of obviously really well cut diamonds with near perfect HCA scores, near perfect BrillianceScope readings, near perfect external and internal symmetry, AGS 0/GIA EX/EX ratings that for all intents and purposes seem to be among the finest diamonds available. I''m talking about diamonds carried by Rhino, Barry, and Robin and Todd. Here are the questions:

1) After running numbers on your system, some of these seemingly perfect/ideal diamonds are getting knocked for pavilion depths of 42.7 or under. Why is that? Do you have a reason for 42.8 being the 1A cutoff? The stats of a number of otherwise 1A diamonds I''ve seen that are below 42.8 pavilion depth seem to indicate there''s not a good reason for suggesting these to be 1B diamonds.

2) On some of the same diamonds, they are falling to 1B or 2A based on girdle readings of "thin to medium"? I''m wondering why this should make a diamond fall out of 1A.

3) I''m seeing ostensibly perfectly cut diamonds with near perfect stats dropping from 1A to 1B or 2A based just on slight variations in pavilion depth and girdle width. However, there does not seem to be a logical reason for this. It makes these AGA class categories seem somewhat arbitrary unless you have a better explanation.
 

oldminer

Ideal_Rock
Trade
Joined
Sep 3, 2000
Messages
6,390
The gradings were not entirely arbitrary, but based on lots of practical observation of real diamonds in the marketplace.

Any diamond scoring 2A or better is likely to be a very nice looking diamond. Now the issue is how large a field should a "top" quality stone be given. The 1A grade is very narrow while the AGS 0 is very large in comparision. My 1B is just abpout the AGS 0 range, but still a bit tighter.

The work of these charts was done long before we had brilliancy reading devices and long before the HCA estimation process. We will have diamonds that have excellent brilliancy outside the standards of Tolkowksy's range, but often they may have faults of durability or diameter. The AGA round diamond chart is Tolkowsky derived and is a safe haven for the careful shopper. There are admittedly alternative makes in diamonds that exist which look very good, but in general, they have issues which have some effect on their cut quality when viewed by an experienced diamond grader.

As far as the 42.8% pavilion depth goes I drew a line in the sand when it comes to a 1A diamond where even a touch of excess depth changes the diameter of the stone. I like to get the full size diameter for my money and not a mm less when I can. It is arbitrary and does leave some very fine looking diamonds out of the 1A grade. That is probably a very good reason that AGS 0 has been so widely accepted. It is a broad grade encompassing the very finest and some stones that are just not the very best, too. A commercial approach was not what I had in mind. I wanted to give graders a tool to make fine separations about "CUT". Appraisers want to split hairs and know "finest" "nearly the finest" "very good" etc. Dealers want all these stones to be sold as "finest". Can't say I blame them. Selling is a war sometimes.

People who use the AGA approach to cut grade will get good advice that is given without a sales pitch. Surely it is conservative advice. Loose advice can create worse problems and unhappy consumers. I have seen it all too often.

We tell consumers that AGS 0 diamonds are all super to very good stones. We tell them AGA 1A to 2A diamonds are about the same range. We just made more gradations. The diamonds are still the same good ones.
 

caratgirl

Brilliant_Rock
Joined
Jan 1, 2003
Messages
634
Hi David,

Just one question. In some of the diamond cutting books I have read, the pavilion angle of 40.75 seems to be extra important to the cut grade, yet you do not have that listed, just the crown angle. Why is that? I am a total amateur, so I am just curious if the rest of the information makes up for that, or other parameters listed. Thanks.


Oh, and another question. The % for a medium girdle seem awfully high - am I reading that wrong?
 

rbjd

Shiny_Rock
Joined
Feb 4, 2003
Messages
154
"As far as the 42.8% pavilion depth goes I drew a line in the sand when it comes to a 1A diamond where even a touch of excess depth changes the diameter of the stone. I like to get the full size diameter for my money and not a mm less when I can."

This doesn't explain why a pavilion depth of 42.7 falls out of 1A class.
 

oldminer

Ideal_Rock
Trade
Joined
Sep 3, 2000
Messages
6,390
When the pavilion percentage falls below 42.7, the corresponding pavilion angle becomes "too shallow". That's why... I hope that gives a more direct response. I mis-read your earlier question.
 

rbjd

Shiny_Rock
Joined
Feb 4, 2003
Messages
154
I've seen many stones for sale with pavilion depths of 42.7 and pavilion angles of 40.7, 40.8, 40.9.

Believe me, I'm not trying to knock your system as much as I'm trying to gain knowledge, but are you saying stones with pavilion depths of 42.6 or 42.7 can't have proper angles or just that they don't generally?

In the case of a stone with pavilion depth measurements of 42.4 to 42.8 (averaging 42.7) and pavilion angle depths between 40.7 and 40.9 (averaging 40.7) I can't see why the pavilion depth being slightly shallow makes much difference unless I'm missing something.
 
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