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Help me understand IGI cut grading please!

LSL

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Apr 11, 2021
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Hi! Most lab diamonds I see are graded by IGI, but I am really struggling to understand what makes an ideal cut?

For example, why is this diamond (https://www.igi.org/reports/verify-your-report?r=LG447063514) an ideal cut while this one (https://www.igi.org/reports/verify-your-report?r=LG455040416) is only an excellent cut?

Isn’t a 40.1 degree pavilion angle really shallow with obstruction problems...? To that end, wouldn’t a 40.5/35.1 combination be much better? Is the cut grading perhaps because the second stone is deeper? I have been coming across quite a few IGI ideals that seem to have worse pav/crown combinations than excellents, am I just missing something?

Any opinions and views are welcome! Thank you!
 

John Pollard

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Hi! Most lab diamonds I see are graded by IGI, but I am really struggling to understand what makes an ideal cut?

For example, why is this diamond (https://www.igi.org/reports/verify-your-report?r=LG447063514) an ideal cut while this one (https://www.igi.org/reports/verify-your-report?r=LG455040416) is only an excellent cut?

Isn’t a 40.1 degree pavilion angle really shallow with obstruction problems...? To that end, wouldn’t a 40.5/35.1 combination be much better? Is the cut grading perhaps because the second stone is deeper? I have been coming across quite a few IGI ideals that seem to have worse pav/crown combinations than excellents, am I just missing something?

Any opinions and views are welcome! Thank you!

Hi @LSL - @denverappraiser was kind enough to ask me to weigh in here.

Based on what I know about the system you guessed it. The 62.5% depth likely moved it to Excellent - it's the only spec that stands out.

RE 59.5/40.1/34.6: Lab approaches differ.

GIA's grading system permits more deep combos that many, because it presumes close viewing, ergo an obstruction metric which darkens shallow stones. AGS uses two obstruction metrics for a cumulative result. Their system gives Ideal to many shallow stones GIA would knock to VG. It's 'center-point' is shallower than GIA's. The IGI metric has a shallower center-point still - as does the HCA.

1621980122919.png
 

DejaWiz

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Wouldn't that shallow of a cut mean it would exhibit a dark and possibly dead center zone (under the table)?
 

Karl_K

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Wouldn't that shallow of a cut mean it would exhibit a dark and possibly dead center zone (under the table)?
Yes in a ring at closer viewing distances in a earring or pendant with the longer social viewing distance they can rock.
85% lowers can help minimize the effect.
In Rings they should have a pavilion in which all the mains are over 40.5 actual angles or 85%+ lowers.
 

John Pollard

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Wouldn't that shallow of a cut mean it would exhibit a dark and possibly dead center zone (under the table)?
It depends on the conditions. That basic TPC combo results in a very bright stone. The problem, as you get shallower, is the diamond reflects back more of what's directly overhead.

Thus: A shallow stone can show balanced contrast at arm's length. Depending on the specifics, however, a viewer drawing closer and closer will cause the contrast pattern to darken and get bigger faster than other combos.

1621982016225.png
 

John Pollard

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Yes in a ring at closer viewing distances in a earring or pendant with the longer social viewing distance they can rock.
85% lowers can help minimize the effect.
In Rings they should have a pavilion in which all the mains are over 40.5 actual angles or 85%+ lowers.
I had the following in my post above, but moved it here in-step with Karl's comments.

The conventional advice on PriceScope is that shallow stones are great for earrings and pendants. It's about viewing distance. It's solid online advice.

That said, shallow isn't as egregious as deep. Across a dozen years teaching cut classes in various jewelry stores I've seen more shallow combos I thought were beautiful, overall, than deep combos - where the darkness problem is systemic leakage.
 

Karl_K

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That said, shallow isn't as egregious as deep. Across a dozen years teaching cut classes in various jewelry stores I've seen more shallow combos I thought were beautiful, overall, than deep combos - where the darkness problem is systemic leakage.
I can go along with that for 2 reasons.
1 obstruction varies with a ton of variables.
2 long lgf% can migrate some of the efects.
 

John Pollard

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I create less obstruction than most.
 

LSL

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@John Pollard @Karl_K @Garry H (Cut Nut) thank you for weighing in on this! Definitely clears the air up a bit. I must say, I did have the opportunity to view that 59.5/40.1/34.6 stone and the arrows were almost always completely dark when viewed close up, with a dim slightly lifeless table. At a distance though it was definitely very bright! But I didn’t see much fire coming off that stone, presumably because of the large table and relatively shallow crown for such a pavilion? Looking at GIA and AGS tables this stone is nowhere near excellent/ideal for both, so as you mentioned IGI must have a much higher tolerance for shallow stones.

Now the deeper stone on the other hand I have not seen in person though its excellent grade stood out to me. I do wonder how this stone will look and I am willing to bet it will look better than 59.5/40.1/34.6 (please holler if you think otherwise)

This stone sits at AGS Ideal and GIA VG if we use its rounded proportions. Of course it’s very likely some pavilion mains are in fact below 40.5 degrees but this is definitely preferable to an average of 40.1 for a ring. To be honest, 62.5% wouldn’t have struck me as sufficient to cast a stone from ideal into excellent, I’d be more wary of say 62.7% if I were reading the report without the cut grade. It’s really interesting that IGI’s tolerance for shallow stones extends really far beyond PS ideals, yet its tolerance for deeper stones seems to be really small!

ETA: now these 2 proportions would have me more worried because of a 62.5% depth, strange CP relationships and possible steep-deeps but... somehow they are ideal cuts?
1. 2.
Definitely falling into the rabbit hole here
 
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DejaWiz

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I'll say this: my wife's newest is a 58/40.8/34.6 with a crown height of 14.5 and a 60.9 depth. Very brilliant...one of the most brilliant diamonds I've seen...and quite fiery on the crown, but most of the fire from the center (arrows) only happens when viewing at an angle, and in very very fast but intense flickers. We were going for a mix of brilliance and fire with a heavier lean on brilliance, so it meets our needs exactly.

if the 59.5/40.1/34.6 that you saw is heavier towards brilliance and spread and that's what you are after, then it might be a good grab.

Ask about their return policy in case you want to test drive it for a couple/few weeks to make sure it exhibits, with no compromises, what you're after.
 
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LSL

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Oh @DejaWiz no worries I am not going after the 40.1 stone! As mentioned while really white it looked rather lifeless in terms of the amount of sparkle it was throwing off. I am really just exploring these out of interest to better understand why/how IGI decides what makes an ideal cut (assuming we are picking a diamond for a ring).

I am in no way criticising a 34.6 degree crown and I’m sorry if it came across that way. When I said that crown was shallow and hence less likely to give off fire I meant in relation to the pavilion angle. A 34.6/40.8 like you have is perfect, but paired with a 40.1...? I don’t even know what sort of crown would be required to be complementary, 37?? (Is this even possible) Perhaps @Karl_K or the others could weigh in but I don’t think there’s much saving such a shallow pavilion.

I was unfortunately not able to see that 35.1/40.5, 62.5% depth stone, which is why I’m interested to know if it is possible that an IGI excellent cut might in fact perform better than an ideal cut in this case.

Based on what I’ve read on here it doesn’t seem like depth has as huge an impact as the crown/pav relationship. The stone might face up slightly smaller and this one is also not a steep-deep so not an issue there.

Though as the other two certificates I dug up show, IGI doesn’t quite seem to have an issue with grading steep-deeps as ideal... so why this stone was graded excellent and IGI’s grading/decision pathway is still confusing (and interesting!) to me
 

Garry H (Cut Nut)

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Oh @DejaWiz n

Based on what I’ve read on here it doesn’t seem like depth has as huge an impact as the crown/pav relationship. The stone might face up slightly smaller and this one is also not a steep-deep so not an issue there.

Light refracts and reflacts based on angles of incidence.
You are correct. Depth % means diddly squat
 
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DejaWiz

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Oh @DejaWiz no worries I am not going after the 40.1 stone! As mentioned while really white it looked rather lifeless in terms of the amount of sparkle it was throwing off. I am really just exploring these out of interest to better understand why/how IGI decides what makes an ideal cut (assuming we are picking a diamond for a ring).

I am in no way criticising a 34.6 degree crown and I’m sorry if it came across that way. When I said that crown was shallow and hence less likely to give off fire I meant in relation to the pavilion angle. A 34.6/40.8 like you have is perfect, but paired with a 40.1...? I don’t even know what sort of crown would be required to be complementary, 37?? (Is this even possible) Perhaps @Karl_K or the others could weigh in but I don’t think there’s much saving such a shallow pavilion.

I was unfortunately not able to see that 35.1/40.5, 62.5% depth stone, which is why I’m interested to know if it is possible that an IGI excellent cut might in fact perform better than an ideal cut in this case.

Based on what I’ve read on here it doesn’t seem like depth has as huge an impact as the crown/pav relationship. The stone might face up slightly smaller and this one is also not a steep-deep so not an issue there.

Though as the other two certificates I dug up show, IGI doesn’t quite seem to have an issue with grading steep-deeps as ideal... so why this stone was graded excellent and IGI’s grading/decision pathway is still confusing (and interesting!) to me


No offense taken, because you didn't come across as criticizing at all...I was merely trying to speak to the shallowness of the one you had looked at and relate my experience with a slightly shallower cut (but still within super ideal specs) of my latest piece in order to give you some insight of what to expect with the 59.5 depth without complementing PA and CA, should you decide to but it.

Alas, I agree with the others...that PA 40.1 will probably not jive with the CA 34.6 at all, and, as Garry just stated, the depth won't play as critical a role, as long as the PA and CA do complement one another. The 40.5/35.1 may be a nice fiery one, but be careful with it since the CA would be more suited to a PA of 40.6-40.7

If a bit more brilliance is what you're after, then sticking with a PA/CA around 40.8/34.3-34.9 would be favorable.

If more fire is what you want, then maybe target a 40.6/35.0-35.3.

These are just rough guidelines based on what others that have experience with thousands upon thousands of diamonds (like Garry) have posted all over the forum.

I find this chart an extremely helpful tool:

Screenshot_20210522-225548.png

Now to speak towards IGI: it's no secret that their grading criteria can be much looser than some of the other well regarded labs, such as AGS and GIA. The only role any grading lab plays is to establish some kind of baseline of categorical measurements that they choose to define, grade a gemstone against those measurements, and provide a report card in order to help sellers achieve top selling price. Nothing is stopping them from re-categorizing, say, DEF color range to a single simplified category called "colorless". But a D commands a much higher price tag than an E or F (if all other measurements are equal), so they stick with what benefits the sale values the most. IGI can muddy the waters a bit, and there are multiple examples of an IGI diamond being sent to GIA and it come back with significantly different gradings. But that also holds true with GIA: there are a few examples out there of a GIA diamond getting different ratings from AGS. This probably has to do with the human error factor: different gemologists will see the same gemstone with different properties. That variation can happen with the same gemologist from day to day...maybe one of them had a rough night and they're a little off the mark the next day. The machines and devices used to take measurements are human-made, human-calibrated, and human-operated, as well. It's not an infallible process.

In my experience, dealing with IGI diamonds requires a good bit more due diligence than the amount of due diligence one needs to exert with GIA or AGS (even GCAL, which seems to be getting more popular) diamonds.

There's nothing wrong with choosing an IGI diamond, as long as the diamond looks and performs how the buyer wants, which is the same advice I'd give to GIA or AGS diamond buyers, anyway.
...meaning, see them with eyes on them in a variety of lighting conditions, or at least vetted by a trusted party. Ideally, do both if possible: go and see diamonds that have been vetted by a reputable consultant/seller. If buying online, you can still do both if the reputable seller has a stellar return policy.

Again, absolutely no offense was taken. We're here to help each other as much as possible with the info that we're given, and that's what I was trying to do, so let me also apologize if I came off as taking some kind of offense... absolutely *not* the case. :)
 

LSL

Rough_Rock
Joined
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Messages
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No offense taken, because you didn't come across as criticizing at all...I was merely trying to speak to the shallowness of the one you had looked at and relate my experience with a slightly shallower cut (but still within super ideal specs) of my latest piece in order to give you some insight of what to expect with the 59.5 depth without complementing PA and CA, should you decide to but it.

Alas, I agree with the others...that PA 40.1 will probably not jive with the CA 34.6 at all, and, as Garry just stated, the depth won't play as critical a role, as long as the PA and CA do complement one another. The 40.5/35.1 may be a nice fiery one, but be careful with it since the CA would be more suited to a PA of 40.6-40.7

If a bit more brilliance is what you're after, then sticking with a PA/CA around 40.8/34.3-34.9 would be favorable.

If more fire is what you want, then maybe target a 40.6/35.0-35.3.

These are just rough guidelines based on what others that have experience with thousands upon thousands of diamonds (like Garry) have posted all over the forum.

I find this chart an extremely helpful tool:

Screenshot_20210522-225548.png

Now to speak towards IGI: it's no secret that their grading criteria can be much looser than some of the other well regarded labs, such as AGS and GIA. The only role any grading lab plays is to establish some kind of baseline of categorical measurements that they choose to define, grade a gemstone against those measurements, and provide a report card in order to help sellers achieve top selling price. Nothing is stopping them from re-categorizing, say, DEF color range to a single simplified category called "colorless". But a D commands a much higher price tag than an E or F (if all other measurements are equal), so they stick with what benefits the sale values the most. IGI can muddy the waters a bit, and there are multiple examples of an IGI diamond being sent to GIA and it come back with significantly different gradings. But that also holds true with GIA: there are a few examples out there of a GIA diamond getting different ratings from AGS. This probably has to do with the human error factor: different gemologists will see the same gemstone with different properties. That variation can happen with the same gemologist from day to day...maybe one of them had a rough night and they're a little off the mark the next day. The machines and devices used to take measurements are human-made, human-calibrated, and human-operated, as well. It's not an infallible process.

In my experience, dealing with IGI diamonds requires a good bit more due diligence than the amount of due diligence one needs to exert with GIA or AGS (even GCAL, which seems to be getting more popular) diamonds.

There's nothing wrong with choosing an IGI diamond, as long as the diamond looks and performs how the buyer wants, which is the same advice I'd give to GIA or AGS diamond buyers, anyway.
...meaning, see them with eyes on them in a variety of lighting conditions, or at least vetted by a trusted party. Ideally, do both if possible: go and see diamonds that have been vetted by a reputable consultant/seller. If buying online, you can still do both if the reputable seller has a stellar return policy.

Again, absolutely no offense was taken. We're here to help each other as much as possible with the info that we're given, and that's what I was trying to do, so let me also apologize if I came off as taking some kind of offense... absolutely *not* the case. :)

Awesome, we are both good then! I’ve definitely learnt a ton from people here and hope to continue to!

Well said regarding the gradings and also that is one handy table! Given what I’ve been seeing from the certificates, IGI stones would definitely ideally need to be seen in person before purchase, or have a very solid return policy. I started this out of curiosity but I am truly discovering more and more strange cases. With AGS and GIA I feel I could guess at the potential grades and not be that far off but IGI is really another matter... I thought depth was the damning factor and then I found those steep deep IGI ideals :lol:

But on the bright side! This does mean that one might be able to get a stone with better proportions for a lower price since they might be graded Excellent instead of Ideal? I can’t even begin to tell you how many I came across with pav angles between 40.1-40.3 AND crowns of 33-34 getting an ideal cut. It’s really quite mind boggling. But perfect for pendants/earrings I guess...
 

DejaWiz

Shiny_Rock
Joined
Apr 23, 2021
Messages
143
@LSL - here are some screengrabs of 0.8 carat diamonds.

Here's what I did:

LAB DIAMONDS
IGI only
VS2 only
Ideal cut only

EARTH DIAMONDS
GIA only
VS2 only
Excellent cut only

Hopefully the screen that you view these on has good color accuracy and contrast, because the differences within the same color grades are obvious - for both IGI and GIA!

First, the lab diamonds. 1st shot is D, 2nd is E, and 3rd is F.

1 Lab 080 D VS2 IGI Ideal.png

2 Lab 080 E VS2 IGI Ideal.png

3 Lab 080 F VS2 IGI Ideal.png



Here are the Earth diamonds, same order for color grades:

4 Earth 080 D VS2 GIA Excellent.png

5 Earth 080 E VS2 GIA Excellent.png

6 Earth 080 F VS2 GIA Excellent.png


As you can hopefully see, there are some obvious (sometimes wild) variations within the same grading ranges. In this case, I have set the search parameters for grading lab, carat, clarity, cut, and color to very specific criteria, but the results are all over the place for color, especially.

Hopefully this provides a clear example of what I meant when it comes to the process of grading gemstones being prone to fallible human error.

It's not something that is a fault in and of the process, but rather something that we need to understand and accept (since we are all human) that does happen during the process in order to increase our own due diligence when subjectively screening a bunch of different diamonds that are candidates for making a purchase.

This approach of understanding human error must also be applied to the other measurements of diamonds: there will *always* be some margin of error for things like the CA, PA, table size, and symmetry, for example. And those margin of errors are why every grading lab does some averaging or rounding. I see a grading cert as a list of "more than likely this" instead of a "definitely this" for the individual measurements and grades defined by whichever lab that a given cert originated.

In other words to make a long story short: What is listed on the cert, regardless of which lab did the measuring and categorizing, may not be purely accurate, which is why my advice is to always see the diamonds in person and with various lighting conditions because, just like humans, every finished diamond ever cut and polished is uniquely different.
 

Karl_K

Super_Ideal_Rock
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Oh @DejaWiz @Karl_K or the others could weigh in but I don’t think there’s much saving such a shallow pavilion.
38 degree crown 40% stars and 79+ lowers 40.1 degree mains.
Outside any expected range for a mrb unless custom cut.
In other words possible but impractical in mined diamonds.
In mmd they are cutting then shallow because the rough says so and when they get rough deep enough for such a thing to be possilbe they are cutting steep deeps just like the mine rough cutters.
 
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Karl_K

Super_Ideal_Rock
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because the differences within the same color grades are obvious - for both IGI and GIA!
Good job putting that together but the most odvious differences is that people are wearing different colored shirts when taking them.
 

LSL

Rough_Rock
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Apr 11, 2021
Messages
25
@DejaWiz thank you for putting these together! What caught my eye first was definitely the higher crowns on some and the uh... more pancake/squashed shape of some others :D definitely a wild variation even within the same cut grade!!
 

LSL

Rough_Rock
Joined
Apr 11, 2021
Messages
25
38 degree crown 40% stars and 79+ lowers 40.1 degree mains.
Outside any expected range for a mrb unless custom cut.
In other words possible but impractical in mined diamonds.
In mmd they are cutting then shallow because the rough says so and when they get rough deep enough for such a thing to be possilbe they are cutting steep deeps just like the mine rough cutters.

Oh golly a 38 degree crown now that would be a sight to behold! I think if I saw a 38 crown on a certificate I might actually ask if it was a typo. The stone itself would look wild as well!
 

DejaWiz

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Messages
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Good job putting that together but the most obvious differences is that people are wearing different colored shirts when taking them.


This is purely speculation on my part, and I am admitting my ignorance on the subject: Since there are no reflections of people captured on any of the frames/angles for those 360 views, it doesn't appear that there are any people present while the spin photography is being captured (such as inside of an Iconasys studio), are there? I honestly don't know, so I'm just asking, whoch will help me learn what to watch for with such photography methods utilized on the web.




@DejaWiz thank you for putting these together! What caught my eye first was definitely the higher crowns on some and the uh... more pancake/squashed shape of some others :D definitely a wild variation even within the same cut grade!!

You're welcome!
 

Karl_K

Super_Ideal_Rock
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such as inside of an Iconasys studio
There is a large opening in the front that is at the right height to catch someones shirt color when it is on a desk in front of them and other light pollution.
Some designs are better at showing less of this than others.
tudio-02.jpg
The diamond sits on the turn table and is controlled by the computer, it turns the set number of degrees then a photo is snapped then it turns again, so on and so on.
Then the software stitches the images together into a "video".
 

DejaWiz

Shiny_Rock
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Messages
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There is a large opening in the front that is at the right height to catch someones shirt color when it is on a desk in front of them and other light pollution.
Some designs are better at showing less of this than others.
tudio-02.jpg
The diamond sits on the turn table and is controlled by the computer, it turns the set number of degrees then a photo is snapped then it turns again, so on and so on.
Then the software stitches the images together into a "video".


That makes perfect sense - thank you, Karl_K!
 
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