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Ideal Depth Percentage for a Round brilliant?

aptp

Rough_Rock
Joined
Oct 29, 2016
Messages
16
I've done fairly extensive research on the ideal proportions of a round brilliant and there seems be consensus on the crown angle, pavilion angle and table % range, but not on the depth % range.

Old PS posts recommend a range, usually between 59-62% and other websites (eg. Paul of Beyond4Cs) recommend 61-62.5%.


Will anything over 62 be too deep, making the diamond look smaller? Why do so many sources stick to the 61-62.5% range- what is it about this range that makes it the best?

Conversely, although sticking to the lower end of the range (59%, 60%) will make the diamond look larger, is this not the ideal range?

Any input appreciated. Thanks.
 

diamondseeker2006

Super_Ideal_Rock
Premium
Joined
Jan 11, 2006
Messages
55,631
Please use these which are what we see most top ideal cut stones measuring:

These are measurements to help you stay in ideal cut territory with a GIA excellent cut stone.

table: 54-58

depth: 60-62.3

crown angle: 34-35.0 (up to 35.5 crown angle can sometimes work with a 40.6 pav angle)

pavilion angle: 40.6-40.9 (sometimes 41.0 if the crown angle is close to 34)
 

flyingpig

Ideal_Rock
Premium
Joined
Nov 7, 2015
Messages
2,175
Some accept 62.5% as long as the diameter is not significantly affected.
60.0% is fine.
I don't think 59.0 % is mathmatically possible while maintaining ideal crown and pavilion angles and keeping the girdle at a reasonable thickness.
Just look at some diamonds with depth of 59.0% or lower. These diamonds often have shallow crown angles of 33.0 or less, significantly affecting fire and sparkle.
 

Dancing Fire

Super_Ideal_Rock
Premium
Joined
Apr 3, 2004
Messages
31,548
I'd like RBs b/t 61-62% depth.
 

Diamond_Hawk

Brilliant_Rock
Trade
Joined
Apr 8, 2014
Messages
1,221
aptp|1482285108|4108694 said:
I've done fairly extensive research on the ideal proportions of a round brilliant and there seems be consensus on the crown angle, pavilion angle and table % range, but not on the depth % range.

Old PS posts recommend a range, usually between 59-62% and other websites (eg. Paul of Beyond4Cs) recommend 61-62.5%.


Will anything over 62 be too deep, making the diamond look smaller? Why do so many sources stick to the 61-62.5% range- what is it about this range that makes it the best?

Conversely, although sticking to the lower end of the range (59%, 60%) will make the diamond look larger, is this not the ideal range?

Any input appreciated. Thanks.
aptp,

You've obviously done some great work and you have certainly come up with a good starting place (the numbers).

No matter what numbers you utilize, remember that you are, ultimately, operating in a 'wire frame' world with these numbers.

Each angle on the lab report is an average of several different measurements that are then rounded. Why is that important? Because two diamonds that have a lab report with identical carat, color, crown, pavilion, table and depth could have differing light performance due to the rounded averages. Now - carat, color, and clarity as graded by any reputable lab will vary only rarely. With cut it is a different matter:

On a GIA report the rounded averages can be impactful. There are 57 facets, each one is a separate measurement and all of them are, essentially, mirrors reflecting light. The number for crown angle, for example, is a rounded average of 8 separate measurements which gets changed to the nearest half degree. So a 34.5 means the average was 34.3, or 34.4, or 34.5, or 34.6, or 34.7 (the average!) on a report showing "34.5" - that's a lot of variables. Now take that same process and apply it also to Pavilion angle, Lower Halves and Stars... I hope you see where this is going.

NOTE: Pavilion Angle is rounded to the nearest .2 - but is a VERY important number on the report. So if you have, say 8 measurements that average to 30.9 in one diamond and that average to 40.1 in another, the report will still say "PA 40.0"- but the 'mirrors' will treat the light differently in the two diamonds - and produce different results.

So, how do you know what diamond to choose if the numbers on the report do not automatically mean it is a top performer?

Once you have found a diamond (or 3) that pique your interest, insist on light-performance images (Ideal-Scopes and ASETs). This will help differentiate which diamonds have the most efficient light return. If you will post the images here on PS the members will help explain what you are seeing and, no doubt, make recommendations for you. Many online retailers (and some brick & mortar stores) will provide these images for you upon request.
 
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