How do you calculate EC cut grade?

[Karl_K]

Karl, your conditions where you disagree are far out. That's why the tool is for SCREENING, not for final selection. I never saw a 4% crown height princess which could compete with a better cut princess with a decent crown height, but maybe in theory one could produce such a stone. The shallow crown angle result would likely be a durability issue and what about weight retention. Is your example a viable commercial reality or a computerized hypothetical with no market reality?

Extremely low crowned princess cuts exist in the market and at least one of them was well cut. Was it exactly 4%? I don't know. Durability will depend on the girdle.
Someone running across one is what set me out researching it.
In computer modeling a "princess" diamond with a 99% table and just a girdle can have surprisingly good performance. It is one of those thing that if I was rich I would have a precision cutter cut just for the fun of it. Until then it will remain virtual.

 

[Karl_K]

The example for the visual size, I used a simple visual size which is the area inside the outline of the diamond face up. It is what consumers understand. You pull out a "3D" area to refute my opinion of fact. That is pure is computer geek stuff and not usable data for marketing or consumer understanding.

Either way 3d or area inside the girdle depth does not equal spread in step cuts.
A 73% depth step cut could win both over a 63% depth step cut.
Emerald cuts are challenging with different l/w ratios and how depth is calculated. It is depth in mm/shortest side in mm.
At lets say 1ct 60% depth a 1.1l/w and a 1.5l/w will have vastly different mm depth.

 

[Karl_K]

No, I can't agree that simply having an extremely thin girdle noted on a GIA report equates to a surety of durability concerns.

I lean towards Dave's thinking on this one and tend to want to eliminate them from consideration.
There are things however you can do to migrate the concerns like protecting that area with a properly made V prong or a bezel where if it isn't broke in setting it should be of no more worry than the rest of the diamond.

 

[Rockdiamond]

Not to belabor the point- this discussion is great.
Many....even most people shopping here that find the guide won't read this discussion.
If someone is shopping with us and they bring up Dave's guide, it prompts a healthy discussion that ultimately produces a more informed shopper.
I couldn't imagine how to put such a chart together- maybe I visualize it differently.
Whatever- people find charts helpful. And I very much respect what you've done Dave.

I agree with Karl on the difficult of using depth percentage to make assumptions. In general anyway. I come across some fancy colored stones with super high depth ratios- in the '90's. Such stones will always look small for the weight. But in "normal" ranges- and below- you can't make valid assumptions till you see the stone. I've seen some amazing EC's in the '40's depth.

 

[Rockdiamond]

I lean towards Dave's thinking on this one and tend to want to eliminate them from consideration.
There are things however you can do to migrate the concerns like protecting that area with a properly made V prong or a bezel where if it isn't broke in setting it should be of no more worry than the rest of the diamond.

SO I'm clear- he's Dave and I'm David?
Karl- your point assumes things and ignores others.
Breaking a diamond is not exactly easy. You'd have to hit it exactly the right way, with the right force. While it might seem that a bezel protects the stone more, it actually puts more metal in contact with the diamond- in some cases increasing the minuscule possibility of breakage.
Same for a V prong.
Any 90 degree corner on a diamond is obviously a concern. If for no other reason it can cut your skin. But you can't protect any diamond from breakage, if it's being worn- no matter the diamond or setting. $hit happens. But not all that frequently if it comes to chipping or breaking diamond.
BTW- in these rare cases of damage- far more likely than a break, is a small chip, which can, in many cases be repaired invisibly.
I'm not saying to totally ignore the girdle- just that many cases of EX THIN do not raise durability concerns

 

[oldminer]

Rockdiamond, I'm saying in my system, "Don't ignore girdle thickness, but don't make a final decision based on it either".
You say directly above, "I'm not saying to totally ignore the girdle- just that many cases of EX THIN do not raise durability concerns". We are saying the same thing. I am giving people guidance in what may be important sometimes while you are saying girdle thickness may not be important sometimes.

GIA and AGS do concern themselves with extremes of girdle thickness in their overall grading, so one can infer there is a reason for them to do so. They don't explain why, but they do it for good reason. Try to sell a very thin girdle diamond to a dealer and see if they won't haggle over the issue and insinuate they need to recut the stone. Every "problem" in a diamond leads to an argument over value.

Both mine and your suggestions would help equally to screen through diamonds in order to get a best selection with the least deficiencies, especially when selecting sight-unseen stones. However, I made this available to consumers in a visible way with an organized process which is easy to access. Getting your advice or some vendor's advice would be far more inconsistent, helter-skelter, and potentially highly biased, especially from competing vendors. Such potentially problematic advice is of far less usefulness and erratic in delivery. I have provided consumers and vendors a consistent approach which I believe makes a credible difference in a positive way.

I put this out in public view long before the rest of the industry felt any obligation to discuss any of the details of cut quality, durability, finish and visual size. Now we, at the very least, have open and frank discussions. These are leading us to honest and consistent methodology which serves to increase consumer confidence.
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The issue over a shallow crown height is not only about the potential of chipping on some stones, not every stone, but also about proper dispersion and scintillation generation back to the viewer's eyes which will not occur as well with little to no crown facet size or shallow angles.

In general, when one makes something that is highly useful 90% of the time, someone always complains about it not working right 10% of the time. If you keep working on this system, over time and with effort to improve it, you get to 95% usefulness, then maybe to 98% or even more. Are we to abandon making strides in the right direction? I don't think we should and no tech person will disagree. Look how much better Windows 10 is than XP or Vista.

Diamond rough costs way too much to create commercially uninteresting diamonds for the sake of experiment. It costs a fortune to promote an odd cut stone such as the Crown of Light which is made by a very substantial DeBeers Sight-Holder. The system that I came up with did not look to the outlier extremes, but centers on finding the "Best" and the "Better" normal and commercial stones in what consumers see as a blind market. If a Princess cut style diamond can be created with a very shallow crown height which will perform like a current AGS000, then why is it not being cut now? It isn't because the economics are not there or highly limited. The market is virtually non-existent. The stone would look strange or unusual and people generally want the comfort of buying what they recognize as "normal".

 

[Rockdiamond]

Hi Dave,
To clarify: GIA does NOT take girdle thickness into account in grading an emerald cut, or other fancy shape- as they don't grade the cut of fancy shapes.
I've never had a negotiation to buy or sell a diamond that hinged on girdle thickness. Ever.
If a stone has girdle damage, yes that is a discussion. But not girdle thickness specifically. In general, if a dealer is concerned about a girdle, they'd pass, as opposed to negotiating based on that aspect, in my experience.

As I mentioned above- there's a segment of the population that will pay attention to the charts, making them feel "safer" buying blind.
My point is that there's also quite a few people that would prefer to get a human observation. Will some of the dealers giving observations be less than honest? Unfortunately, yes. But from my perspective, it's far easier for a consumer to use knowledge of human nature, and also the internet, to check out a seller. Plus, they'll be looking at pictures. To me, the entire idea of using a numerical chart to rate diamonds overlooks the most important elements, while fixating on aspects that might be totally unimportant.
An Emerald Cut with an EX thin girdle can be far more desirable than one with a Medium Girdle.
Do you see a lot of consumer stones with Girdle damage Dave? I don't.

 

[Rockdiamond]

The issue over a shallow crown height is not only about the potential of chipping on some stones, not every stone, but also about proper dispersion and scintillation generation back to the viewer's eyes which will not occur as well with little to no crown facet size or shallow angles.

This statement simply has no basis in useful fact. We'd need to assess the entire diamond, including pavilion to make any sort of assessment. Plus dispersion and scintillation are entirely subjective in real life.

 

[Squizabel]

While it might seem that a bezel protects the stone more, it actually puts more metal in contact with the diamond- in some cases increasing the minuscule possibility of breakage.
Same for a V prong.

Can you explain why this would be the case? Do you mean during setting, or during everyday wear?

(sorry to thread jack OP)

 

[Rockdiamond]

Hi Squizabel ( what a kewl name!!)
It's not all that easy to chip a diamond. You have to hit it with a reasonably hard object from exactly the right angle, with the right amount of force.
The metal of the bezel is in contact with many spots on the diamond. If you strike the diamond on something, it's possible to exert the right amount of force where the metal chips the diamond.
A prong can also do this.
That's considering potential damage during wear- you asked about setting as well.
Both prong and bezel require the setter to exert force on the diamond with metal.
A bezel presents a bit more risk during setting as compared to a prong, in my experience.

But overall, the risk is low because in both setting, and wear, a specific chain of events and forces need to occur. There's really no way to predict when it might occur. Diamond is, after all, the hardest substance. Doesn't mean it can't break- just not all that easy.

 

[Squizabel]

Thanks Rockdiamond - learn something new every day!

 

[oldminer]

GIA does take girdle thickness into account on rounds and when they do grade fancy shapes, I'd be willing to wager they will extend it to such stones. One rarely sees extended chips on medium to thick girdle stones. Thinness of the girdle combined with shallow crown angle does result in a chipping tendency. Shallow crowns often come with shallow crown angles, but GIA does not provide consumers with fancy shape crown angles, so the screening tool is useful to eliminate that doubt. More parametric data from GIA might be provided, but that is not up to me.

You might find an emerald cut with an extremely thin girdle which is far nicer looking than a similar stone with a thicker girdle, but like I suggested, the thin girdle is a negative in durability but the negative might be outweighed, for you, if the stone looks wonderful. For those who seek "the best", then the ext thin girdle is a finish fault and they might wish to eliminate the stone from their list. Its their choice. Would you not prefer a medium girdle stone over an ext thin girdle stone if both were of equal beauty, shape, weight, color, clarity, and, transparency and fluorescence? For myself, the fact that girdle thickness is meaningful with rounds tells me it should also be that way for fancy shapes, too. AGS does consider this important for 000 ratings on all stones they cut grade. There seems little doubt that girdle thickness has some impact in correctly screening for better stones in the process of elimination.

 

[Karl_K]

I think we can all agree when it comes to diamond breakage they are just plain weird.
A lady got mad at her husband with good cause. Then took off her ring and slammed it down to the concrete and stomped on it, kicked it across the ground them stomped on it some more. The setting was totally destroyed but the diamond was like brand new, 0 damage. I saw the ring afterwards but not the event itself but I believe the story of how it happened.

A different lady dropped her ring a couple inches into a sink and half of the diamond shattered into powder. I saw the diamond afterwards.

Both were princess cuts.

Weird eh?

 

[oldminer]

Below is what AGSL uses for grading rounds, cushions, princess and oval shapes. AGSL has a great reputation, but does not have the world-wide audience like GIA does. The traditional diamond trade is foot dragging on quantifying and standardizing how well diamonds are cut. Partly, this is legitimate because there are many lovely diamonds which will become less desired if they can no longer be safely represented as "Excellent" cut stones. Delaying the recognition of what makes various shapes excellent allows sellers to keep doing what has been working for decades.

The following is slightly modified but taken direct from the AGS Lab website explanation of what they consider in the cut grade of round, oval, cushion and princess cut diamonds: Note that I have made 5, 6 and 8 in bold as those are the factors Rockdiamond and Karl have responded about.

The Idea of Performance and the Factors Comprising the Light Performance Grade


Of all the factors that contribute to the appearance of a diamond, what are the most important? What are the most universally agreed upon attributes of finely cut diamonds? What are the key elements of light performance? The answers can be reduced to four properties that are regarded almost universally as the essential characteristics of fine makes:


1. Brightness. Finely cut diamonds tend to appear BRIGHT in a broad variety of lighting environments.


2. Fire. Fine makes tend to exhibit large amounts of FIRE when illuminated in fire-friendly lighting environments.


3. Leakage. Fine makes do not have excessively large areas of light LEAKAGE.


4. Contrast. Fine makes will have enough dark/light CONTRAST to be interesting, but not too much dark as to impact the brightness of the stone.




Besides the four factors listed above, there are five other performance factors that encompass the AGS Light Performance Grade. These are more practical concerns related to the diamond’s performance and also have analogues in other types of performance grading systems in other industries:


5. Weight Ratio. Finely cut diamonds achieve a high level of performance without using excessive weight.


6. Durability. Fine makes are resistant to chipping.


7. Tilt. Fine makes do not exhibit the “fish-eye” effect.


8. Girdle. Fine makes do not have excessively thin or thick girdles, mostly due to durability/weight ratio concerns.


9. Culet. Fine makes do not have an excessively large culet.




Additional considerations of the AGS diamond Light Performance Grade:


10. Polish


11. Symmetry

 

[Karl_K]

AGSL can not and does not grade patterns so it is missing a huge part of the picture with fancies EC/SE in particular to stay of the topic of EC cuts.

 

[oldminer]

EC/SE in particular to stay of the topic of EC cuts.

What are you saying here? Is there some kind of typo, etc because I don't understand the abbreviation or the rest of the sentence.

 

[Rockdiamond]

Hi Dave,
The first thing to clarify in light of the title of this thread- there's NO workable, widely accepted cut grade for emerald cut diamonds.
Possibly AGSL has one on the books, however, it's totally irrelevant in the real world- as is the rest of their fancy shape grading. The only shape where there's a glimmer of recognition for AGSL fancy shaped cut grading is Princess Cuts- and even that is on the fringe of acceptability in the trade.
A big part of the reason is the underlying assessment of "light performance". There's just no broad agreement in the trade, or with consumers, on what's the "best" princess cut cut.
Besides that, #6, in particular, is totally incorrect.
Fine makes are NOT resistant to chipping inherently.
In fact, we could easily make the case that a heavy, terribly cut stone would be more resistant to chipping than a fine make.
That also renders #8 invalid as well.
To your question-

Would you not prefer a medium girdle stone over an ext thin girdle stone if both were of equal beauty, shape, weight, color, clarity, and, transparency and fluorescence?

Dave- in the real world, I've never had such a situation come up when shopping for an emerald cut.
With round diamonds, there's so many to choose from ( in most color/size/clarity combos)- and they're all so close in appearance ( round is round, there's no "different" shaped round), it makes sense to look at minutia.
But even in round diamonds, there are times that a given stone possesses a rare combination of color, carat size and clarity that choosing a VG over an EX may indeed be warranted.
Moving back to emerald cuts- consumers are not going to have the choice of equal stones.
I know we'll never agree on this Dave- but the reason the cut grades hinder consumers is the fact they eliminate or downgrade stones that should not be downgraded based on factors which are not definitive- such as GIA girdle grades used without observation.

 

[Karl_K]

What are you saying here? Is there some kind of typo, etc because I don't understand the abbreviation or the rest of the sentence.

AGSL can not and does not grade patterns so it is missing a huge part of the picture with fancies in particular Emerald Cuts and Square Emerald cuts to stay on the topic of Emerald Cuts.......
Its the same with any other system for that matter.

Sorry it was not clear.
My shoulder is in so much pain its hard to hold my arms up to type sometimes so my typing is worse than usual lately. I am in a hurry to get it out so I can put my arm down.
It is worse after PT so expect really poor typing tonight if I post :{