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AGA Round Billiant Cut Grading Chart

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zoebartlett

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I''m determined to understand how to find quality diamonds (RB specifically). I''ve read the tutorial here and on GOG (several times), and both are extremely helpful in understanding the basics. I think, however, that I have a mental block on understanding the percentages and the grading chart. Can someone please explain the chart below?

http://diamonds.pricescope.com/round.asp


I was reading another thread from a long time ago and I found Aljdewey''s post that included a picture of a diamond. Can someone please explain that?

https://www.pricescope.com/community/threads/diamond-iq-quiz-for-newbies.11497/

Thanks!
 

Finding_Neverland

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I am curious,........... In diamonds in general,............

Why don''t the %''ages of the crown, girdle, and pavillion add up to 100% top to bottom??

Like the table is 53%, 55%, or whatever % of 100% of the width/diameter?? Why isn''t the height the same?? A proportion of 100%??
 

JulieN

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Crown, girdle, pav and depth % are related to the diameter, as well.
 

Richard Sherwood

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Date: 1/21/2007 12:30:51 AM
Author: Finding_Neverland
Why don''t the %''ages of the crown, girdle, and pavillion add up to 100% top to bottom??

Like the table is 53%, 55%, or whatever % of 100% of the width/diameter?? Why isn''t the height the same?? A proportion of 100%??
All percentages are based on the diameter. The table is such & such percentage of the diameter, and the depth is such & such percentage of the diameter, and the crown/pavilion depth is such & such percentage of the diameter, and the girdle thickness is such & such percentage of the diameter.

Your crown height + girdle thickness + pavilion depth will add up to the total depth percentage if you take into account the girdle thickness at the "peaks", which many labs don''t. The European labs and AGS have begun taking this into account, but many other labs are still reporting the girdle thickness at the "valleys", which will yield a lesser total depth percentage (usually about 1.7% less).
 

zoebartlett

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Garry, in regards to your post: "your question is too broad. You do not understand %''s, or angles? or charts?", I guess I need help understanding how to read the grading chart in general. I''ll start with what I think it''s telling us but please point me in the right direction if I''m wrong. I apologize for being broad in my earlier post.

If one were thinking about purchasing an AGS Ideal Cut, I''d look at "class 1A" and "Class 1B" and then, depending on which to go with, I''d follow the chart vertically. I''d make sure that, if for example, I wanted a class 1A, the table should be 53-58%, the crown angle should be 34.0-34.7, the crown height should be 14-16.3%, and so on, ending with the polish and symmetry being "Excellent-Very Good."

If I wanted a Class 2A or 2B (International fine trade cut), I''d do the same thing - follow the chart vertically, just as I did above. I''m assuming that class 3A and 3B are good choices as well to consider, but class 4A and 4B are below average, so those are ones to be wary of.

How am I doing so far? Way off base or on the right track? How does one determine whether to go with Class 1A or 1B, or 2A or 2B or 3A or 3B?

I''m trying...

http://diamonds.pricescope.com/round.asp

 

Finding_Neverland

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Date: 1/21/2007 12:42:21 AM
Author: JulieN
Crown, girdle, pav and depth % are related to the diameter, as well.
Duh! Did I have a blonde moment there or what??!!


I just couldn''t figure it out while looking at the diagrams. Why diamonds were only 55%, 60%, whatever deep. I thought there''s no stupid questions except the ones not asked. Maybe I was just too sleepy to think. It was late here.

Then this morning, it came to me. Depth %''age was a function of the diameter!! Eureka!!
 

Finding_Neverland

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Date: 1/21/2007 1:12:30 AM
Author: Richard Sherwood
Date: 1/21/2007 12:30:51 AM

Author: Finding_Neverland

Why don''t the %''ages of the crown, girdle, and pavillion add up to 100% top to bottom??


Like the table is 53%, 55%, or whatever % of 100% of the width/diameter?? Why isn''t the height the same?? A proportion of 100%??
All percentages are based on the diameter. The table is such & such percentage of the diameter, and the depth is such & such percentage of the diameter, and the crown/pavilion depth is such & such percentage of the diameter, and the girdle thickness is such & such percentage of the diameter.


Your crown height + girdle thickness + pavilion depth will add up to the total depth percentage if you take into account the girdle thickness at the ''peaks'', which many labs don''t. The European labs and AGS have begun taking this into account, but many other labs are still reporting the girdle thickness at the ''valleys'', which will yield a lesser total depth percentage (usually about 1.7% less).
Thanks much, Mr. Sherwood. For the info about the girdle thickness. Peaks and valleys. I will definitely keep that in mind!!
 

tanalasta

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You have to realise those cut charts are fairly arbitary - and slightly outdated. Cut-charts are forever going to be updated one day.

More importantly, you should pick the ''best'' cut. But even then, there are combinations of angles within the say a specific cut-range that may not complement each other. The ''No Go Zone'' as cutnut tends to put it. An ideally proportioned diamond stands a fantastic chance of being a great stone. However, only a visual assessment, both with the naked eye and tools such as an Idealscope/ASET etc... and/or the AGS ''light performance'' grading can confirm this.

The lazy person with a lot of money can simply buy a Cartier or Tiffany stone that is not going to look dull and drab. It may be far from the ''best'' stone available (especially at the price) but certainly would be a very pretty stone.

All the learning on PS is more about excluding crappy, dull, drab stones ... once you get to ideal cuts, you really begin to split hairs. If you want to save yourself some time, just buy an AGS000 that you''ve seen and loved or one of the whiteflash ACA stones that someone else has seen and would assure that you would love.

If you want to hunt for a stone yourself, you need to read the FAQ section (or I wrote a buying guide recently in the rocky talky section for the sake of it - just do a thread search of my posts). And you are more than welcome to post a stone''s Idealscope Image and proportion / certificate for people on this forum to post a comment.
 

oldminer

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Of course the charts are "fairly arbitrary". All charts of this sort, nearly all "grading" schemes are arbitrary measures humans put in place to make hopefully valid relative assessments. They are never going to be outdated, but there is simply going to be more ways to make measures, projections and more specific grades in the future. Diamond is a natural, or synthetic material with absolutely consistent characteristics of durability, refractive index, hardness, brittleness, luster, etc. These many unchangeable features dictate cutting ranges for proportions and ALWAYS will. Yes, we will have increased variety with new software and cutter's tools, but only within the confines of the nature of the material which is never going to change.

The AGA Cut Class charts are not whimsy. They were the result of many years of observations within the diamond business. They have been subjected to total market scrutiny and have been slightly refined and revised dues to valid criticism from experts from all over the world. Even Octonus, the makers of Helium, built the AGA Cut Class grades right into their software years ago and teach it to gemological students at Moscow State U. The 700 plus appraisers in the National Assoc of Jewelry Appraisers adopted these charts years ago in an effort to bring consistency of grading to their work.

The charts will never be perfect. We don't, and can't, measure every parameter of importance on every single diamond. There is endless variety. This is why all tools that attempt to predict Light Behavior from measuring physical parameters will be somewhat invalid or inaccurate. They may be "good enough" for now, but later on, they may be passed by by better technology. The AGA Cut Class charts provide a valid way to screen diamonds of all the common shapes. You can find wonderful looking diamonds in the 2B and better categories, but most "Premium Cut" diamonds will be found from 2A and up. The 1A category, especially in round diamonds is very strict to Tolkowsky specs. Maybe I should have made it broader for commercial acceptance, but no one has said it is a wrong grade for top stones. There are some wonderful diamonds a bit outside the range, but when we talk "screening" I believe there is a lot of usefulness to be taken advantage of.

Besides, its free. Free can't be as bad as paying for the same advice.....
 

zoebartlett

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Thanks Oldminer and Tanalasta!

I have read your tutorial, Tanalasta and I actually replied to your post. I had been putting together something similar on just the basics for my boyfriend to look at. Anyway, I have been practicing judging whether a diamond is good quality by reading posts from people. I know that looking at images if available are very helpful, as is inspecting a diamond with one''s own eyes. If I''m correct, the table should typically fall within the 51-57% but can go as high as just over 62% and still be a quality diamond. Total depth should be from 58-63%. Those are the specs. I have been focusing on.

Tanalasta, you had written,"there are combinations of angles within the say a specific cut-range that may not complement each other. The ''No Go Zone'' as cutnut tends to put it." What are some combinations to stay away from? What angles would be acceptable? This might not be easy to answer right now because I don''t have anything specific in mind to run by you. I just want to learn as much info. as possible. It''s fascinating to me.

If you were to recommend further reading that goes into depth about any of this, what would it be?
 

Finding_Neverland

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