What are the perfect diamond proportions?

[doanuts]

In general people say that the overall geometry is responsible for the optical performance. So there is a range of proportions that would be classified as ideal, but what is the exact proportions to make the most brilliant diamond. If you assume absolute precision in cutting, there must be an answer. Anyone know what it is for a round shape diamond?

 

[diamondseeker2006]

Here's the cheat sheet for rounds (compiled from various experts on PS):

depth - 60 - 62%
table - 54- 57%
crown angle - 34- 35 degrees
pavilion angle - 40.6- 41 degrees
girdle - thin to slightly thick, thin to medium, etc (avoid very thin or thick)
polish and symmetry - very good and above

note - with crown and pavilion angles at the shallower ends ( CA 34- PA 40.6) and steeper ( CA 35- PA 41) check to make sure these angles complement in that particular diamond - eyeballs, Idealscope, trusted vendor input - check as appropriate!

Also, configurations depend on each other. A little give here can still work with a little take there.

With that said, here''s a "Cliff''s Notes" for staying near Tolkowsky/ideal angles with GIA reports (their numbers are rounded): A crown angle of 34.0, 34.5 or 35.0 is usually safe with a 40.8 pavilion angle. If pavilion angle = 40.6 lean toward a 34.5-35.0 crown. If pavilion angle = 41 lean toward a 34.0-34.5 crown.

GIA "EX" in cut is great at its heart, but it ranges a bit wider than some people prefer, particularly in deep combinations (pavilion > 41 with crown > 35). In other words, there are some GIA Excellents that are better than others.

Also, always check diameter to be sure stone faces up as large as it should, ex: 1 ct. 6.5mm, 1.5 cts. 7.4mm, etc.

 

[Rockdiamond]

There are no "perfect" dimensions.
The "cheat sheet" posted above is merely the opinion of someone. And it's different than the proportion set used by either GIA, or AGSL- and many experts as well.

The problems in answering the "ultimate" brilliance question are many- foremost- how do we test which diamond is most brilliant?
Then, next question- is brilliance the goal?
What about contrast and patterning?
If we want brilliance exclusively, it's going to look like a light bulb. if it reflects back 100% of the light going into it, it's a mirror.

So- the best cut could be pretty much anywhere inside GIA's EC cut grade- or, within AGSL labs 0 cut grade.
Any divisions within these categories are opinion nothing more.

 

[Lula]

Perfect diamond proportions -- ah, depends on who you ask around here you'll get a variety of answers.

This article will give you a detailed description of how different diamonds of the same cut grade can look to the naked eye, under the Idealscope, etc. I found this article useful in helping me sort out how the different combinations of proportions look in an actual stone:
https://www.pricescope.com/journal/laboratory_cut_grades_what_report_doesn’t_show

That article never hyperlinks correctly for me! You can find it on this page:
https://www.pricescope.com/articles/Diamond+Grading?page=1
Laboratory Cut Grades: What the Report Doesn't Show.

There are other excellent articles about diamond cutting on that webpage as well.