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Great Cut usingTable & Depth Percentages

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riley

Rough_Rock
Joined
Jan 20, 2003
Messages
23
When reading a GIA Certificate will I know that the Crown and Pavillion angles will be right (per Tolkowsky) if the Depth % is 54% to 63% and the Table % is 58 to 62%?

I want to make sure that the diamond reflects light correctly to get the maximum brilliance, fire, etc. It''s hard really see what''s going on with bright incandecent lights (which most jewelers have) as opposed to being out in natural light (which jewelers don''t want you to do for obvious security reasons).
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
Hi Riley,

As is so often the case with GIA-certed diamonds, it's going to be tough to give any meaningful opinions as to how the stone will perform without knowing some more information about it. As respected as GIA certs are, they really only give some of the information that is useful to the likes of RockyTalk contributors like us, and that's where other tools often have to come into play. If you are able to get the dealer to obtain Sarin or Megascope reports for the diamond you're considering (some dealers will arrange this for customers), or shop from a source that already includes that information, you will have a far easier time using geometry to predict the quality of the diamond's cut. Just using the GIA-provided information you're not going to be able to tell too much. Tolkowsky used proportions and angles when doing his study, so it is necessary to know this information if you're trying to get a diamond that falls within true "Tolkowsky ideal" parameters.

-Tim
 

Mara

Super_Ideal_Rock
Joined
Oct 30, 2002
Messages
31,003
Unfortunately the answer to your question is pretty much a big NO.

The reason is that those ranges may seem helpful but in fact they are very broad, AND just because a stone has parameters within those ranges, does not mean that that particular combo will be a nice stone.

For example, we've seen some stones on here with Depth of 61 and Table of 60 which look horrible under the scopes and when plugged into the HCA. Also my stone is a 61.4 table and a 56.8 depth, not exactly Tolowsky Ideal but a very brilliant stone and gets a 1.4 on the HCA. No one would consider my stone remotely close to ideal, in fact AGS rates it a 7 because those parameters give it a very shallow crown angle, but the #'s are well within the range specified in your post. Similarly, chances are that a stone with a 62% table and a 54% depth would be very shallow. On the same token, a stone with a 58% table and a 63% depth might be too deep and the stone would look smaller than it really is.

So you see there are many combos within those ranges which will yield non-ideal or non-desirable stones. I have heard it said that some stores recommend to get a TRUE ideal stone that you stay within the ranges more along the lines of Table: 56-58 and Depth: 59-61. Some will say never to go deeper than 61 on depth. Some will say never to go above 59 on table.

There will be many opinions and preferences, but as Tim notes, the crown and angle pavilions will help muchly in determining the proportions of the stone in relation to the table and depth, and then also the HCA will let you know what it thinks of your stone (and warn against too deep or too shallow stones). In the end it's mostly about what your eye likes and sees..and that will also help you rule out stones that are not up to your specs.

Good luck!
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
14,919
Read the tutorial
and paste the link to see the photo's
https://www.pricescope.com/tutor_60.asp

There is a strange fallacy that if a diamond has proportions of 60% depth and 60% table then it will be a beautiful diamond. It’s a great idea, but it simply does not work.

Here are two extreme examples of bad proportion combinations. Both are 60:60.

The pavilion angles on these vary from 36 degrees to 45 degrees and this range of variance actually exists out there. Of them all only the very small range between 39.7 degree and 40.4 degree pavilion angles would be worthy of consideration.
 

RockDoc

Ideal_Rock
Joined
Aug 15, 2000
Messages
2,509
All of the facets are important ..... There is a lot more to a stone being ideal than just the table, crown, and pavilion mains.

Unfortunately, consumers believe that if the numbers are "ideal" or an AGS 0 or gets a low number on the HCA that this means they will get a wonderfully performing diamond.

This simply is not factual.

Had a stone in the lab today.... great numbers AGS 0 cut grade, 0.50 on HCA.. and it was a dog as far as performing goes.... Low to medium in brilliance.


I know everyone wants an easy do it yourself method to analyzing stones, but it doesn't work on all the time.

I'll be posting more on this stone soon...

Rockdoc
 

RockDoc

Ideal_Rock
Joined
Aug 15, 2000
Messages
2,509
What about the other 40 facets? Don't they matter.

NOTE: THERE ARE NO STONES ACTUALLY BEING CUT TO TOLKOWSKY standards.

Tolkowsky didn't range the table... he said 53%!

Further, the total depth he indicated is shallow because the Tolkowsky stone had a razor thin girdle.

Tolkowsky did this in 1919 or therabouts.. don't you think we've advanced a little technologically since then?

In addition, the numbers or proportions don't always equal a great performing diamond.

Saw a stone yesterday that performed very well with 59% depth and 64 table.

Today we checked an AGS 0 cut grade stone, that didn't do well in the performing side at all.

Each stone is an individual, unique stone, each performs the way mother nature "tells" them to.

If the stone doesn't perform, the numbers don't mean much- do they?

Rockdoc
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
14,919
For the range that Tolkowsky would have predicted had he had a computer go to http://www.folds.net/diamond_girdle/index.html and http://www.folds.net/diamond/index.html#software


This may come out as gibberish?
2% girdle
Pavilion Crown Table Total Depth Ratio
39° 37° 17' 0.556 0.405 + 0.169 + 0.017 + girdle - culet ht = 0.611 - culet ht
40° 35° 17' 0.558 0.420 + 0.157 + 0.017 + girdle - culet ht = 0.613 - culet ht
40° 45' 33° 27' 0.558 0.431 + 0.146 + 0.018 + girdle - culet ht = 0.614 - culet ht
41° 32° 45' 0.558 0.435 + 0.142 + 0.018 + girdle - culet ht = 0.614 - culet ht
42° 29° 15' 0.557 0.45 + 0.124 + 0.018 + girdle - culet ht = 0.612 - culet ht


Read the whole of Tolkowsky at http://www.folds.net/diamond_design/index.html#ed_note_02
 

optimized

Shiny_Rock
Joined
Dec 28, 2002
Messages
306
Garry,

I stumbled across that online version of the Tolkowsky book a month or so ago myself. Very cool that it's available on the Internet. I was especially appreciative for the calculations that modified the original knife-edge girdle model to account for the fact that just about no diamonds today use knife-edge girdles. A handy, if very dry, reference.


RockDoc,

I'd also be interested in hearing some more about your atypical diamonds...

-Tim
 
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