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What Are The Ideal Proportions For A Round Brilliant?


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Can anyone tell me what the ideal ranges are for a round brilliant in terms of depth and table percentages, and crown and pavilion angles. I'm getting mixed information from different websites.


I'm looking to purchase at least a 1.5 ct round brilliant with GIA cert triple excellent, at a minimum F color and VS2-SI1 clarity with a budget of $15k.


Thanks for any advice!

Edited by jkad
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You're gettng conflicting information because it's not that easy a question and not everyone uses the term 'ideal' in the same way. The two major grading authorities, GIA and AGS are quite different. GIA doesn't use the term at all, and in the AGS system it's not a function of either table or depth percentages. Depth isn't even one of the variables used. Crown and pavilion angles play into both and are used similarly. You mentioned that you want a GIA xxx. One of those x's is for ther cut grade and that's their top grade. I don't count it as the end of the question but they don't include enough information on their reports to subdivide it further in a meaningful way.


The $15k budget should be fine if you're shopping online but the local stores are likely to have a problem with it.

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Thank you for your reply, Neil. I'm just starting to learn about all these aspects of cut grading, so please excuse my ignorance and multitude of questions! :blink:


Should I focus on crown and pavilion angles instead of the table and depth percentages; it seems like a combination of both? Or is relying on GIA's xxx system good enough? I've reviewed GIA's and AGS's cut grading tables, but could you recommend a general range that I could input into the various search engines online for the percentages and angles?


I've also used the HCA on pricescope.com, plugging in the numbers with diamonds that I've found online. Is there any parallel to the cut grading and the HCA results, or does HCA measure entirely different aspects of the diamond? Is HCA reliable, as I've read that there are some limitations to it.


Do you have a preference on which grading authority to use? GIA seems the safest bet but I'm not quite knowledgeable about AGS standards. I am looking to purchase the diamond online given my budget and just want to make sure I'm getting a good deal, and I want to make an educated decision before buying, especially since I'm buying without actually seeing it.


By the way, your posts are very informative and appreciate the time you're taking to reply!

Edited by jkad
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Thanks. I'm glad I could help.


I wouldn't say the HCA is entirely different from the GIA system but it's a subset and it analyzes things in a somewhat different way. For GIA graded stones that include a cut grade I would ignore it entirely as a shopping tool. For GIA graded stones WITHOUT a cut grade (pre 2006) it's moderately useful but I would wonder why the report is so old and I would avoid non-GIA and non-AGS stones entirely for other reasons so it doesn't really matter with those. That makes it of pretty limited use for your objectives.


AGS standards have become ridiculously complicated and there's no good way to do it would a sarin machine, a computer and a bit of software although you can approximate it with some charts with table/crown/pavilion choices. Basically, if you want a stone cut to AGS standards, buy an AGS graded stone.


Personally, I bought my bride an AGS stone (which wasn't all that long ago. I'm engaged but not yet married)


There are variations within the GIA-x range and you can do a little bit with Idealscope/ASET imagery but mostly the key is to actually look at the stone(s). I do find reflector images to be useful but they're not nearly as useful as people wish they were. Identifying a difference between two stones is not the same as flagging one as 'better' than another and if you don't have a benchmark for what different things look like in 'real life', they can be a bit misleading. That's a taste thing and it's the reason that the GIA ranges are a bit on the broad side. AGS 'ideals' are much more similar to one another and I do like the look but this is not the same as saying they're better.

Edited by denverappraiser
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I agree with Neil 100% - but let me try a slightly different take on the question.


If what you are interested in is the cut grade - be it GIA Excellent or AGS Ideal - then why worry about the geometry that produces it? Look at the report, and there it is. You want both? Some vendors and appraisers have the equipment (Sarin scanner and AGS software) to get you an AGS cut grade estimate on a GIA graded stone, and you can get a pretty good estimation of the GIA grade for free using the FacetWare site (https://www.gia.edu/facetware) on an AGS graded stone - though it's a pretty good bet that if it is AGS Ideal it will be GIA Excellent.


If what you are interested in is getting something that you find most beautiful, there is no solution other than looking. The angles, the percentages and proportions produce different results and you may like best something that the average person (which is what GIA is trying to represent) or the average AGS member find not-quite-so attractive. From this point of view, think about selecting the vendor first, and the diamond second. Particularly if you are going to work remotely, work with someone who can look at the diamonds for you, not just someone who will ship them to you from a third party location. The ability to take good quality images, videos and to provide you with commentary that goes beyond "nice" or "awesome" is also a (significant) plus. Believe it or not, just applying these simple criteria restricts the field to relatively few vendors.


Get yourself familiar with the different aspects of light performance (scintillation, brightness and fire), so that you can identify what you see/like and know how to describe it. These three GIA articles are a good start:






Best of luck in your search, and please do keep this thread alive with questions and updates!

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