40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown?????

[tenbinko]

Hi everybody.

I've read several posts by experts in this forum that a 41.0-deg pavilion would go better or be quite safe with a 30.4-deg crown, but from I've read on countless AGS certificates for the super ideals on the market, including BGD signature H&A and White Flash's ACA, there's often a 40.8- or 40.9-degree pavilion paired with a 34.8- or 34.9-degree crown, which seems contradictory to what all experts have been saying about the inverse relationship between pavilion and crown angles.

But if those are the super ideals on the market and are considered the best cuts possible, why is it that the coresponding crown angle is not closer to 34.0 degrees instead?

My guess is that the steeper crown is needed to perfect the H&A pattern, though in terms of light performance, Tolkowsky's 40.75-deg pavilion and 34.5-deg crown combo would still work the best? And a 40.9-deg pavilion with a 34.0-deg crown would be better than a 40.9-deg pavilion with a 34.9-deg crown, in terms of brilliance, fire and scintillation?

Any clarification would be much appreciated.

 

[diamondseeker2006]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

You can easily have a 35 crown angle with a 41.0 pav angle and end up with a stone with ideal light performance. I limit my personal searches to 34-35 for crown angle and 40.6-41.0 on the pavilion angles, and any combinations in there have the potential to be great.

 

[Karl_K]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Brian Gavin set the specs for both brands and he likes the ~40.8/~34.8 combo with his chosen set of minor facets.

Excellent looking h&a images are possible in all sorts of proportion sets.
Some of which will have very high performance some of which wont.

I am rather partial to 56t 41/34 with longer lower halves and if I was building a brand it would be in that range with the highest optical symmetry aka h&a and the tightest angles possible.

So whose idea of perfect proportions is more right?
Both combinations can be stunning as can many other combos.
In person there would be a slight personality difference depending on lighting but one would be hard pressed to call it a performance difference in very well cut samples of each combo.

 

[Laila619]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Personally, I don't like a 40.9 pav. The reason for that is because 40.9 is just an average; in some spots the pavilion could be 41 +, and the stone could possibly be leaky. So to be on the safe side, I look for a shallower pavilion, more like 40.7. If I had a 40.9 pav, I'd rather have a crown around 34-34.5. But this is being really picky, and the reality is that we're waaaay over analyzing this stuff.

 

[Karl_K]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

40.9 is just an average; in some spots the pavilion could be 41 +, and the stone.

Good point, what is in the averages is just as important as the averages.
But if you have IS/ASET images that helps eliminate those that go over thresholds in the average.
Or you can get a full sarin/helium scan.

 

[chicam]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Brian Gavin signature stones are beautiful beyond belief. Whatever those angles that make his Signature are, are to me, the best.

 

[tenbinko]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

With all other things being equal, what's the visual difference between a 40.9/34.8 combo and a 40.9/34.0 combo? Just that the former is a bit more fiery, because of the steeper crown, but no discernible loss in brilliance?

I understand how the lower girdles impact the performance of a diamond, but I'm not sure how the two combos above differ, especially if the LGHs (radious measurement, per GIA) are kept constant at 80% and the stars at 50%.

Any clarification would be appreciated!

 

[John P]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

With all other things being equal, what's the visual difference between a 40.9/34.8 combo and a 40.9/34.0 combo? Just that the former is a bit more fiery, because of the steeper crown, but no discernible loss in brilliance?

I understand how the lower girdles impact the performance of a diamond, but I'm not sure how the two combos above differ, especially if the LGHs (radious measurement, per GIA) are kept constant at 80% and the stars at 50%.

Any clarification would be appreciated!

With "all other things being equal" any differences will be minimal in human terms, unless you're in some odd/specific light condition where a 0.5% crown height makes a notable difference.

As you alluded, the overall level of dispersion seen will depend on different factors. Beyond the primary angles reported there is the amount of deviation from average, all-important minor facet details, brillianteering/indexing particulars and the level of 3D cut-precision.

It's worth remembering that a steeper CA does not necessarily mean a higher crown on its face. 55T/34.0CA, 56T/34.5CA and 57T/35.0CA enjoy the same crown height. Well, technically 55/34.0 beats the others by a micron or so, but for practical purposes they're the same.

 

[tenbinko]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Thank you, John.

But if the minor facets and such are kept constant and the symmetry is perfect, which I know is impossible, but theoretically, what happens one when compares a 40.9/30.8 diamond to a 40.9/34.0 diamond? I thought the latter would more or less resemble the Tolkowsky ideal cut, with little noticeable change in brilliance or fire, but with a 40.9/34.8 combo, theoretically, what would be the change in performance? I thought the increase in crown angle would increase fire...? But this stone would have more weight retention?

You mentioned crown height... I'm just curious, for fire, crown height is more important than crown angle?

Thanks!

 

[John P]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Your questions have diverse and interesting aspects, to me, so I'll take them one at a time.

...theoretically, what happens one when compares a 40.9/30.8 diamond to a 40.9/34.0 diamond?

Crown angles circa 32.5 degrees and lower are worrisome in terms of chipping/durability, so I'd not recommend a 40.9/30.X configuration regardless of performance implications. Technically, with lower halves and stars mid-range (75-80/50-55), perfect 3D cut-precision and normal indexing a 40.9/30.8 would be extremely bright in common sizes, but would have noticeably less visible colored flashes when compared to a modern Tolkowsky make with same minor facet, precision and indexing details.

I presume when you say "perfect symmetry" you mean 3D cut-precision and not facet and proportions symmetry as graded by the labs, correct? It's a long-time bugaboo to me that the word "symmetry" is applied to meet-point symmetry, facet symmetry, proportions symmetry, optical symmetry and physical symmetry. They are all different animals. It's why I have used the term "precision" to imply 3D cutting symmetry and (with cheers) some peers and labs I work with have begun using that term in-house as well.

...with a 40.9/34.8 combo, theoretically, what would be the change in performance? I thought the increase in crown angle would increase fire...? But this stone would have more weight retention?

RE Performance: If we were to gather 10 diamonds with similar table sizes, mid-range minor facets - some cut to 40.9/34.0 and some to 40.9/34.8 - I will dare any of us to decisively identify which are 34.0s and which are 34.8 using naked eye performance observation. While the “hard” Tolk angles are always given as 40.75/34.50 there is a “near-Tolkowsky” range of table sizes and angles that (combined with appropriate other geometry) produce similar optics in a balance which many people find appealing. There is also human physiology involved on the viewer’s side. Eyesight and perception vary – meaning some people tease out more differences than others. So while the technical data points might point to a tic more dispersion versus brightness in diamonds with 34.8 versus 34.0…how “real-life” visible this will be to most humans is potentially negligible and definitely arguable.

RE Yield: With same-table size the weight difference between a diamond with 40.8/34.0 and one with 40.8/34.8 will be 1%. However, if you change the plan from 60/40.8/34.0 to 53/40.8/34.8 that jumps to 4% - which can be useful when you have a primary stone bordering 3 or 4cts and room for the crown.

But in the mainstream planners frequently work to get two diamonds from one octahedron. In these cases it is more practical to use lower crown heights to maximize the primary and the "toppie" (the secondary diamond). This is a reason 60-60s became a popular choice; they're a bright make, the large table and low crowns better-follow the outline of an octa, and planning two 60-60s typically yields more than planning diamonds with higher crowns.

When GIA started grading cut in 2006 producers changed some things: With fewer angle-combos capable of GIA EX in 60-62% tables producers started targeting tables in the high 50s with complimentary angles which would yield the most from the rough. This includes a scandalous combination of 41.6/35.0 paired with tables 56-58, as well as other eyebrow-raising (to me) combos in a borderline “steep-deep” area that is permitted to be GIA EX.

You mentioned crown height... I'm just curious, for fire, crown height is more important than crown angle?

They work together. Table size and crown angle combine to create crown height. Greater crown heights can optimize fire, as long as the angle of the bezels pairs correctly with the pavilion mains…the table and lower halves complement each other…the indexing is appropriate to the overall configuration (interestingly, some steeper combos benefit from slight painting) and high 3D cut-precision promotes larger dispersive “fans” so that the average human pupil clips more fans, resulting in the eye seeing more color in the diamond’s performance. While knowing height is useful, it cannot tell the whole story until you take all factors together.

 

[yssie]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

John, I know I expend considerable quantities of snark on cheatsheets and whatnot, but I have to say (again, as hopefully I've said it before) that I love your detailed responses to stuff like this - thank you for taking the time to post them They're interesting to me because in the details are points that I hadn't considered, or hadn't thought important enough to consider - and that just creates more questions!

I think I remember reading an article from U of Moscow on colour perception a long time ago... maybe it's floating around on here somewhere? It's the first thing I thought of.

*Edited

 

[John P]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

John, I know I expend considerable quantities of snark on cheatsheets and whatnot, but I have to say (again, as hopefully I've said it before) that I love your detailed responses to stuff like this - thank you for taking the time to post them They're interesting to me because in the details are points that I hadn't considered, or hadn't thought important enough to consider - and that just creates more questions!

I think I remember reading an article from U of Moscow on colour perception a long time ago... maybe it's floating around on here somewhere? It's the first thing I thought of.

*Edited

Yssie, thank you very much for the mention. I'm grateful to be able to stimulate thought, just as posters such as you do for me. It's appreciated - and makes me want to keep participating here.

RE Color perception: If you do find that article please link it. I'd love to read it.

 

[bunnycat]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

I'd like to thank you as well. Your posts are so detailed and interesting and I am really enjoying the discussion and what I'm learning.

 

[tenbinko]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Thank you, John, sincerely.

I learned a lot from your response.

I must admit and apologize for the fact that I had extremely limited time on the computer when I posted my last message, and accidentally made a typo... I meant only 34.0, not 30.4, but you kindly responded to exactly that and actually helped me learn something I didn't know before.

But I see now... between 40.8/34.8 and 40.8/34.0, there's little difference... I think according to AGA, 34.8 or 34.9 would not be a top grade (1B instead of 1A), but Wikipedia also says they've been criticized for being overly strict.

Thanks again!

 

[John P]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

bastetcat and tenbinko ~ Thanks for your comments. I’m glad to help.

...I must admit and apologize for the fact that I had extremely limited time on the computer when I posted my last message, and accidentally made a typo... I meant only 34.0, not 30.4, but you kindly responded to exactly that and actually helped me learn something I didn't know before....

No apology necessary. Regardless of typo it was a valid question in the context of this thread. It was refreshing to touch on parameters that are outside normal paradigms.

But I see now... between 40.8/34.8 and 40.8/34.0, there's little difference... I think according to AGA, 34.8 or 34.9 would not be a top grade (1B instead of 1A), but Wikipedia also says they've been criticized for being overly strict. Thanks again!

Correct again. In the big picture AGA and HCA are both centered a bit shallower than AGSL, which is centered shallower than GIA.

Choose any single system and at the borderline of mutual “overlap” their cutoffs are a bit different. In this case 34.8 sits 0.1 of a degree outside AGA’s 1A. But if you were at the shallow side of AGA’s strict standards you’d be outside of AGS0, which is outside of GIA EX, with the shallow side of HCA being outside the others. On the opposite side GIA EX permits deeper combinations than AGS0, the HCA and AGA.

I’m actually glad the systems are independent of each other, as it permits insight into “how” and “why” they have been arrived-at.

As it relates to being at the 0.1 degree border of a strict system: I’ve seen diamonds scanned two or three times where results of Table or CA become slightly different. ( Trivia question…can anyone guess why those two elements are subject to most scan error? ). Your basic numbers are great and I would move on to the more important question; how do all the facets work together? Then you've moved from the predictive world into actual performance, where the rubber meets the road.

 

[tenbinko]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

That's really interesting, John, I didn't know that the same diamond scanned a couple of times could yield different numbers... if it's different labs we're talking about, I understand, but if it's the same equipment from one lab, then I don't understand... I can't guess why the specs would change, as I know too little about the scanning technology that's used... but only the table and crown angle, eh?

 

[John P]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

That's really interesting, John, I didn't know that the same diamond scanned a couple of times could yield different numbers... if it's different labs we're talking about, I understand, but if it's the same equipment from one lab, then I don't understand... I can't guess why the specs would change, as I know too little about the scanning technology that's used... but only the table and crown angle, eh?

Every non-contact scanner has tolerances. The most common given error is ± 0.2 degrees angular and ± 20 microns linear. Some scanners are better, some worse, but even the same scanner may report slight differences when a diamond is run multiple times. These are "perfect condition" tolerances. The differences will increase if you fail to calibrate/update hardware and software and don't keep a clean environment.

As an example, I used a diamond cloth to clean this stone just now (normally I do much more) and did not decontaminate the stage. In my scanner's screen-shot you can see particles present on both diamond and stage. The stage is especially influential if particles occur where the edge of the table meets the platform. When this happens the scanner "sees" the particle as an extension of the diamond, creating table size and/or crown angle errors. Unless you are a fastidious stage-cleaner these will inevitably occur.



Given the same scanner, someone running multiple scans without calibrating or cleaning enough will have less accurate results than someone who is fastidious, which means more time dedicated to each scan. And even then no scanner is perfect.

Going further, remember that many data points given on a lab document are already averages of multiple individual facet measurements...and at GIA even the averaged numbers are further rounded up-or-down. This is one of the reasons lab documents can be considered good indicators of basic data, but the most fastidious "cut geeks" will investigate things much farther, locally.

It's important to mention that we are discussing a high level of technical detail here, one which many people including pros will never care-about, but I believe it's in the spirit of this thread.

 

[tenbinko]

Re: 40.9 PA goes better w/ 34.8-34.9 crown or 34.0 crown????

Thanks, John, that makes things so much more clear now.

And I've saved the picture to my computer.

 

[Gemly]

Amazing thread!!