Post subject: Re: Diamond proportions quiz with prize!
Posted: Sun Jan 15, 2012 7:52 pm
Gold Member
Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
That was most interesting. Now, let me suggest how one can "cut grade" any round brilliant diamond from its certificate data, without the necessity of even looking at it. All one needs do is determine the ratio of its crown angle to its pavilion angle and the value of its total depth. The closer the ratio of C/P approaches the value of 0.84 and the total depth to 61.3% of the stone's diameter, the higher the stone will be rated. Try it, you'll like it. It's called the Hanneman Objective Cut Rating System.
Post subject: Re: Diamond proportions quiz with prize!
Posted: Tue Jan 17, 2012 7:01 pm
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
Bill Hanneman wrote:
That was most interesting. Now, let me suggest how one can "cut grade" any round brilliant diamond from its certificate data, without the necessity of even looking at it. All one needs do is determine the ratio of its crown angle to its pavilion angle and the value of its total depth. The closer the ratio of C/P approaches the value of 0.84 and the total depth to 61.3% of the stone's diameter, the higher the stone will be rated. Try it, you'll like it. It's called the Hanneman Objective Cut Rating System.
I'm afraid that such rule doesn't work. You need to take into account also the table diameter. Here are two stones that accomplish that rule, one ideal cut and the other with very wrong proportions. Both have C/P angle ratio and the total depth very close to the proposed values.
Ideal brilliant cut: C/P angle ratio = 0.85, Total depth = 60.5%
Very wrong proportions, but: C/P angle ratio = 0.84, and same total depth = 60.5%
Egor can you make renderings of both stones to show more clearly what the effect would be?
Addendum: I think we must stay within reasonable angles to have this type of rule, work. Also renderings will make it easier to grasp for members not familiar with looking at gem drawings or even real diamonds very often. Your example number two would lack the dispersion which I like so much in diamonds. Personally I prefer a smaller table and a slightly higher crown than is considered optimal
Post subject: Re: Diamond proportions quiz with prize!
Posted: Wed Jan 18, 2012 6:34 pm
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
Ok Conny, here they are. First the ideal cut diamond and than the other one. Probably the second one won't be so ugly in real life, 45º pavilion angles act as a mirror (see ray-trace video), so one just will see his own inverted reflection. As rendering program (DiamCalc) includes a shadow to reproduce observer’s head effect on lighting, so you just see black mains in this rendering.
Here is a simple ray-trace video for both stones (also DiamCalc facility):
Post subject: Re: Diamond proportions quiz with prize!
Posted: Thu Jan 19, 2012 6:46 am
Valued Contributor
Joined: Wed Oct 22, 2008 5:00 am Posts: 344 Location: Finland
This is classic diamond book mistake I have never seen a SRB diamond with 45° pavilion angle, have you? I think it's non existent, but lives in literature. Now show me a rule without exception…
Post subject: Re: Diamond proportions quiz with prize!
Posted: Thu Jan 19, 2012 8:21 am
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
What I intended to demonstrate is that the rule doesn't work, and for this I used a stone with very exaggerated bad proportions. There is no any limit for the rule, it also doesn't work for smaller angles deviations.
The rule have an error on its basis, if you increase crown and pavilion angles ON THE SAME TIME, the C/P angle relation will be the same, and you can maintain the same total depth (other condition for the rule) simply reducing the crown height. That will cause larger tables.
The same way you can reduce both angles same time and correct the total depth increasing the crown height, causing smaller table.
That's why you need to talk about the table size to use that rule.
And, coming to this point, I'm thinking about the Occam's razor...
Post subject: Re: Diamond proportions quiz with prize!
Posted: Sun Jan 22, 2012 4:13 pm
Gold Member
Joined: Mon May 14, 2007 11:00 am Posts: 1133 Location: Monterey, CA
Conny Forsberg wrote:
Thanks Egor! Dr Hanneman, please comment on the rule and angle extremes.
Barbra, before I begin, I think you might consider switching this subject into the category for Diamonds, as it will eventually evolve into a discussion of diamond grading and how it should be done. You might also want to archive it for the future. This is the beginning of my campaign for a completely objective cut grading system. ----------------------------- To begin, let me say everything Egor has said is technically correct, however, I consider most of it to be irrelevant. I shall explain, but please forgive me if I am wordy.
The pragmatic purpose of grading a diamond is to relate the quality of the stone in question to the asking price. The current use of terms like Good or Excellent is completely unsatisfactory, as these terms are arbitrarily defined differently by each proprietary grading system.
Granted, computers are wonderful calculators, but they can only calculate as they are programed, i.e., to use the algorithm developed by its creator. Today, these algorithms are so complex, no one can easily explain what, why, and/or how everything fits together to produce the final assigned grade.
In an attempt to visually illustrate the significance of each grade, each different grading system arbitrarily draws irregular “concentric” areas on an X-Y graph and assigns a grade to each different area. Usually, the crown angle and the pavilion angle are used as the axes. Unfortunately, such a graph can treat only two variables at a time, and there are many more variables. e.g., crown height, pavilion depth, total depth, table size girdle thickness, facet lengths, polish, symmetry, and culet size.
Since each of these variables must also be considered in this algorithm, its complexity becomes overwhelming and easy understanding becomes impossible. Consequently, the working gemologist is simply forced to pick one proprietary system and, without understanding the whys or hows of it, use it and its terms. Of course this leads to the situation where one system calls the cut of a stone “Excellent” while another calls the same stone “Very Good”. Such schemes are subjective rather than objective.
This simple fact explains why the area designated “Excellent” on the GIA graph is considerably larger than the area designated “0” by the AGS and why GIA grades are considered less stringent. Actually, the fundamental reason for this, in this case, is AGS grades are based on a girdle thickness of 3% while GIA’s tolerance for girdle thickness is 4.5%, a value some 50% greater and a variable not addressed by any of those charts.
Even more important, is the fact one cannot adequately describe the cut of a diamond by the use of a single term, e.g., Very Good. The likes of Brightness, Fire, Scintillation, Polish and Symmetry easily trump the virtues of a stone having optimum Proportions. This means the hallowed term “Four C’s” should be abolished as being inadequate and replaced by something like Five, Six or Seven C’s.
We should really start all over and look at the situation in a pragmatic, purely objective fashion. As one willing to this, I can easily explain why I consider the concerns of Egor to be irrelevant.
The reader should clearly understand I look at this problem from the point of view of a professional appraiser who must assign a monetary value to a diamond and be able to defend that judgment in a court of law. One must also recognizes time is money and one can not afford to spend too much of it on unnecessary activity. ----------------------------- To start, we must divorce the subjective attributes such as Brightness, Fire, and Polish from the objective attributes of Proportions and Angles. This is easily accomplished by dividing all diamonds into one of two categories. I call them FINE diamonds and COMMERCIAL grade diamonds. This decision can be easily and almost instantly made by one with the experience every competent appraiser has.
A FINE diamond shows no remarkable features related to its polish, symmetry and/or girdle, and exhibits an even pattern of bright and dark areas with strong contrast which guarantees an attractive appearance. (I believe this is the GIA criteria for their Excellent grade.)
A COMMERCIAL grade diamond will show remarkable defects related to its polish, symmetry and/or girdle, and exhibits an uneven pattern of bright and dark areas which make its attractiveness a completely SUBJECTIVE matter. (I believe this is the GIA criteria for their Good grade.)
The problem lies in differentiating the intermediate region which is often called Very Good. The need for any such subjective decision is obviated by the Hanneman Objective Cut Rating System which encompasses the terms C/P and Total Depth. By means of a target, it allows one to determine precisely how close a given stone approaches the center of the bull’s eye (0.84:58.9).
All of which brings us back to the original request to “comment on the rule and angle extremes”. The simple answer is that all Fine diamonds plus a lot of stones of lesser quality will be found within the crown angles of 30 to 38 degrees and pavilion angles between 40 and 42.5%. Any diamond outside those boundaries will always show easily noticed features related to girdle thickness, crown angles or height, pavilion angles or depth, table size, polish, symmetry, or an uneven pattern of bright and dark areas which automatically makes it a Commercial grade stone and therefor irrelevant.
One might note, a diamond may indeed have a perfect shape, place dead center in the bull’s–eye, and still be classified as Commercial, if it has a poor polish. Of course, such a condition would be instantly obvious on first examination.
Once a stone has been deemed COMMERCIAL on the basis of shape, there is no real need to examine it further. Any judgement relative to its beauty or attractiveness is simply a subjective opinion. Its value is NOT a function of its Cut Grade, but rather is a function of its Kernel Weight. That, however, is another story. In this case the stone’s value would be very close to that of a similar fine diamond, as repolishing would have little effect on its kernel weight.
Post subject: Re: Diamond proportions: Discussion after Quiz
Posted: Sun Jan 22, 2012 4:52 pm
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Joined: Sun Oct 16, 2005 12:22 pm Posts: 19992 Location: San Francisco
Bill Hanneman wrote:
Barbra, before I begin, I think you might consider switching this subject into the category for Diamonds, as it will eventually evolve into a discussion of diamond grading and how it should be done. You might also want to archive it for the future. This is the beginning of my campaign for a completely objective cut grading system.
Post subject: Re: Diamond proportions: Discussion after Quiz
Posted: Sun Jan 22, 2012 5:55 pm
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
I've read the explanation of Dr. Hanneman with attention but I still don't understand why the data given by me is irrelevant. A new criteria for diamond proportions grading were proposed, to "cut-grade" any round brilliant diamond from its certificate data, without the necessity of even looking at it". I showed that these criteria are incorrect, or at least they need additional parameters to take into account. I think these considerations are VERY relevant for every person who could be thinking to use the proposed rule.
We can talk a lot on different diamond grading systems. For all of them, the first stone shown as example will have Excellent proportions grade, and the other one Poor (or equivalent). And other important point: one shouldn't mix proportions with finish (symmetry and polish), they should be graded separately.
I do understand the basics of Dr Hannemann's proposal. As said, it is pretty obvious if a stone is really out of boundaries with eyesight only. Stones "within boundaries" could be graded by the formula. Polish, Symmetry and Proportions together, is what we judge as Cut, hence the possibillity to make grading easier. That is, if one wish to do it simpler. Many "oldtimers" are actually already using that method even if it is not done by "the Hannemann formula" but rather by a trained eye and a good loupe. As long as we make things more and more complex with different criteria for the same grade there will always be discrepancies between judgment from different Labs.
The KISS principle could very well be applied also on diamonds if there was a wish for it. Btw, I do myself, when grading diamonds, use the HRD-method as being thought to me but I would like to make it simpler...
Egor I know... we all recognise a well cut round brilliant when we see one but as usual no possibility of doing stuff easier. That's why I stick to the HRD nomenclature.
Personally I wish there was not so much fuzz about cut grading, but then of course, I am one of those who love a nicely polished OMC or OEC.
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