What is the effect (if any) on brilliance of >61.0% depth?

[james41]

I have been on a site operated by Fred Cuellar. He is adamant that where a diamond''s total depth exceeds 61.0% it has a dramatic effect on sparkle etc. He reckons light reflection drops to 38% or some such figure.

Is this correct? It seems counter-intuitive for there to be a massive drop off at one particular percentage figure but then again I am not an expert. If he is right there would be a significant difference in appearance between similar stones but with 60.8% and 61.2% depth. I know there are generally degrees of quality and shades of opinion but he does seem to stick by his hard and fast rule. Any informed opinions would be greatly appreciated!

 

[Garry H (Cut Nut)]

This guy would not be allowed to trade in my country.


Do a google search or search this or any forum. Include words like convicted felon in your search.

 

[valeria101]

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On 5/19/2004 6:59:40 AM james41 wrote:



If he is right there would be a significant difference in appearance between similar stones but with 60.8% and 61.2% depth.

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I have no idea... wether this is right or wrong. It is an incomplete statement anyway. Depth alone cannot determine the optics of a diamond, so more info is needed on what happens with the other proportions when this change occurs - it is not clear what "similar" means.

If this is all that is said, it is a very schetchy statement at best, indicating that carving a niche for yourself in any market is profitable, but saying precious little about diamond optics

 

[Magnum]

It is NOT correct. when I first started looking for diamonds, I also found his site and read a lot of stuff in it. It's tough because some things he points out are true, like the fact that if you "buy shy" (a diamond just below a magic number like 1.0, 1.5, 2.0 carts, etc.) you can get a better deal, but it's tough to find diamonds in these weights because cutters will strive to meet that magic number, or if they don't, the stones are snatched up quickly. The fact that there's some truth mixed in with false statements makes it difficult to pick out what to believe. Two theories that do not hold water in my opinion are the "warped diamond theory" (diamonds don't warp, and if you read goodoldgold's tutorial there's a good explanation on why Fred is wrong) and the "61% factor." When I first read about the 61% rule, I remember that he gave a mathematical explanation of how he came up with that number, but I didn't know enough about the values he was plugging into the equation (like the crown and pavilion angles, and table percentage, etc.) so i couldn't really critique it. Being a former physics major in college, I enjoy the challenge of trying to prove or disprove an equation, so after I learned more about the ranges for ideal cut diamonds, I tried to go back and find the equation he used, but couldn't find it on his site. I guess he might have realized that by showing the method, it would allow people to shoot holes in it. As others have pointed out, total depth does not directly affect light performance and there is no sudden drop off in light return in stones that are just over 61%. Pavilion angle and crown angle, followed by table percentage, are the biggest factors that affect light performance. Ideal cut stones with awesome light return can even have total depth that's slightly larger than 62%. obviously, as you get slightly deeper, the diameter gets slightly smaller, for a gvien carat weight, but you don't necessarily have a decrease in light return. Hope this helps, and if anyone knows where on his site he spells out his method for coming up with 61% rule, let me know. i'm still curious.

 

[madmarlin]

Hi. I'm not a diamond expert. Nor do I have all the formulas handy to calculate light return, etc. All I can offer is the following:

I just bought a new stone for my wife. The total depth on the stone is 61.5%. And All I can say is that it sparkles like crazy. Now, of course, I'm biased, but every time I look at it I'm amazed at it's sparkle and brilliance in any light. And in my buying process I looked at many stones before choosing this one.

So, to this untrained eye and opinion, all I can say is that my 61.5% sparkles just fine, thank you very much Mr. Cuellar.

 

[Magnum]

Ok, I went back and actually found an explanation, although not a good one. I thought I had seen a more mathematically complicated one listed before, but I could be wrong.

Question:
I have never seen the 61% Depth rule stated anywhere wlse. In fact, I have looked at diamonds with over 61% depth and cannot tell the difference. There seems to be very few diamonds available with under 61% depth. Is this a GIA standard or a standard you have derived based on your standards?

Answer:
It wasn't my standard it was Tolkowskys. If you look up the original measurements for the American Ideal, you will find a maximum crown height of 16.2%, maximum girdle thickness of 1.7% and a perfect pavilion depth of 43.1%. When you add them up you get the extreme allowable at 61%. If you want mathematical proof, read R.W. Ditchburns book "Light". As far as not being able to see a difference, well the average person can't tell the difference between a CZ and a diamond. Does that mean we should switch to CZ's? Finally, if over 61% diamonds are so grand why isn't there a jeweler any where in the world who will bond them?

And then in a different spot he says the following, but doesn't actually spell it out, even though it can be done very easily:

Once the total depth percentage exceeds 61%, it can be proven very easily by taking the tangent of the crown and pavilion angles and their corresponding crown heights and pavilion depths to show how light enters critical angles in the pavilion of the diamond and leaks out to create a fish eye in a round and deep bow-tie shadows in pears, marquise and ovals.

 

[Magnum]

Even if you believe everything he's saying and accept his numbers of 43.1% pavilion depth and 16.2% crown height as being relevant, that means that if you just increase the girdle thickness from 1.7% to 1.8%, you're going to have a total depth of 61.1% and according to him it's like walking off a cliff and your light return is going to drop from 88-91% to 32-28%. there's no way changing the girdle from 1.7% to 1.8% will have that kind of effect in light return, not to mention all the other flaws in his logic and the fact that properly functioning sarin machines can be off by more than .1% which could determine whether you are over or under 61%. Anyway, hope this helps dispell some of the credence of his 61% rule.

 

[valeria101]

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On 5/19/2004 1:47:07 PM Magnum wrote:



And then in a different spot he says the following, but doesn't actually spell it out, even though it can be done very easily:

[...] taking the tangent of the crown and pavilion angles and their corresponding crown heights and pavilion depths to show how light enters critical angles in the pavilion of the diamond and leaks out to create a fish eye in a round and deep bow-tie shadows in pears, marquise and ovals.


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This is funny! If anyone would actually take the 5min pain to do this, he'd end up plugging the numbers in the HCA to make the math quicker

Well... at least for rounds.

 

[Shay37]

Had to edit cause I messed up the quote thingy. Anyhoo, I thought that a shallow cut diamond produced a fisheye, and a deep cut produced a nailhead? Yet the "esteemed"

Mr. Cuellar claims that it's a deep cut that produces that in this quote here. Hmmmmm.

Shay


Once the total depth percentage exceeds 61%, it can be proven very easily by taking the tangent of the crown and pavilion angles and their corresponding crown heights and pavilion depths to show how light enters critical angles in the pavilion of the diamond and leaks out to create a fish eye in a round and deep bow-tie shadows in pears, marquise and ovals.


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[Richard Sherwood]

Light return performance is largely determined by the relationship of the crown angle to the pavilion angle.

You can have a diamond with a thick girdle which causes the depth to exceed 61%, even 62 or 63% (or more), but if the crown/pavilion relationship is correct, it will be a stunning diamond with a superior light return performance.

 

[james41]

Thanks for the very helpful comments on this issue. It seems to me that the answer is that there is no absolute 61% rule - it is necessary to look at the various angles to determine the position.