Questions for experts here hope will get an answer for it.....

[DiamDude]

I have done the research and tried to learn many things from price scope and wanted to understand the calculations for the same. As previously on HCA there were only options to select carat weight, crown angle pavillion angle, Table, and depth. At that point, very good cut diamonds were also counted in 1.2 and 1.4 HCA scores and I believe later on this got updated and added to select cut also that is a great thing to update it by price scope.

I still have a question on criteria now wherein if the table is 55 and depth is 63 or 63.5 still diamonds are coming up with score range of 1.4 and below that and if I see this diamonds would have girdle thickness and from my point of view ASET won't come good.

So the major factor that I see to bring a great score over HCA is crown and pavillion angle only if they are 34 and 40.6 or 40.8 that will give you a great score even if the table or depth is not the range that is mentioned or experts says.

I hope someone sees that and can come up with a new model of HCA that will actually help people here, I personally follow HCA and I am a big fan of it but sometimes even great diamonds are been declined.

If you guys can work on girdle thickness and show that score that would be great as the girdle plays an important role here. Even previously people used to suggest a crown angles of 34 to 35 now even 32.5 to 35 crown with 40.6 and 40.8 Pavillion diamonds are great.

If any expert here can justify this and can explain it to me it would be great as I am a learner and I have learned a lot here.

 

[Karl_K]

a: girdle size does not have a large impact on diamond performance with most combinations other than an impact on spread. The looks like score covers this and then some.
.
b: HCA number is an indication of complimentary crown and pavilion angles for the table size.
.
c: There are GIA VG combos that get under 2 hca scores.
The reason is that GIA only considers combinations for ring use where hca gives good scores for pendant and earring combinations.
.
d: there are GIA VG combos that could get AGS0 from AGS depending on the specifics.
This is primarily because the GIA system likes steeper pavilions and hates shallower pavilions compared to the AGS system and way more than the HCA.
.
e: There is a super wide range of angles and table sizes that are complimentary with 34 crowns and 40.6-40.8 pavilions. Some of which I would recommend and some not but for other reasons than being complimentary.

If I missed something let me know.

 

[Karl_K]

I see it does not give you a looks like size if you just enter the angles and % without the mm dimensions.

Also if you ask 5 different experts you will get 5 different answers some will vary a bit and some a lot so expect some variation.

 

[DiamDude]

a: girdle size does not have a large impact on diamond performance with most combinations other than an impact on spread. The looks like score covers this and then some.
.
b: HCA number is an indication of complimentary crown and pavilion angles for the table size.
.
c: There are GIA VG combos that get under 2 hca scores.
The reason is that GIA only considers combinations for ring use where hca gives good scores for pendant and earring combinations.
.
d: there are GIA VG combos that could get AGS0 from AGS depending on the specifics.
This is primarily because the GIA system likes steeper pavilions and hates shallower pavilions compared to the AGS system and way more than the HCA.
.
e: There is a super wide range of angles and table sizes that are complimentary with 34 crowns and 40.6-40.8 pavilions. Some of which I would recommend and some not but for other reasons than being complimentary.

If I missed something let me know.

A: I wouldnt agree that girdle thickness does not impact the light performance. If you try using the HCA calculator on a diamond with a very thick girdle, it will give you a pop up that girdle is thick and it wont give you score for the same as it does play some role in light refraction.

B:HCA more precisely gives score based on crown and pavilion angle. Even if you get diamond with 62 table that is higher and depth 58 or 59 still you will get score nearly by 1.5 that means HCA is more related to crown and pavilion instead of other parameters. For the above specs you can search diamonds on pricescope and you will notice that.

C: I would agree with you on this as well.

D:This is honestly the first time I heard that GIA very good can be AGS0. That somewhere means AGS is more lenient than GIA. The popular belief is that AGS is the most stringent when it comes to grading.

E:If you can come up with measurements and girdle part along with more angles to it then scoring would be far better.

Also, if people are going to get 5 different opinions from 5 different people, how can they rely on any expert to shortlist the right stone?

 

[Karl_K]

D:This is honestly the first time I heard that GIA very good can be AGS0. That somewhere means AGS is more lenient than GIA. The popular belief is that AGS is the most stringent when it comes to grading.

Depends on who is right in most cases AGS would be right in my opinion because they take all the facets into account rather than grossly rounded numbers.
That 2 different systems using 2 different ways of clarifying disagree in some areas is to be expected.
Some will just stick to combos they both agree on and others will explore other combos.
Neither system is without flaws.

 

[Karl_K]

Also, if people are going to get 5 different opinions from 5 different people, how can they rely on any expert to shortlist the right stone?

In any field you will not get universal agreement on all points. There are always different viewpoints.
Progress is made because of those disagreements its not a bad thing.
When it comes to MRB diamonds the basics are fairly well understood.

 

[DiamDude]

I think my point was about HCA score not about AGS and GIA please review 2 diamonds below one of them is in range table wherein other is not as per HCA guidelins or expert opinions when ever they provide perameters to any one for search both diamonds shows same HCA even though table and depth are differnt and girdle the only thing that is similar is crown and pavillion. Can we say that HCA is only calculated based on that only.

Diamond | 3.250 ct J VS1 Round Cut Loose Diamond AGS Certified | AGS104096937001 | AGS

This fabulous 3.250 carat Round Ideal cut diamond has a clarity grade of VS1 and color grade of J.It is a great choice for those seeking the finest in diamond cut. | 114304278 | AGS

www.whiteflash.com

Diamond | 3.055 ct G VS1 A CUT ABOVE® Hearts and Arrows Super Ideal Round Cut Loose Diamond AGS Certified | 4453327

This Whiteflash A CUT ABOVE® super ideal cut hearts and arrows 3.055 carat G color VS1 clarity loose diamond is graded by the American Gem Society (AGS) Laboratories. | 4453327

www.whiteflash.com

 

[Karl_K]

A table of 57 and 55.8 are both in the acceptable range in pretty much all systems.
I can not discuss those specific diamonds much beyond that due to board rules.

 

[Karl_K]

This may be what your looking for:
The primary numbers driving the hca score are table%, crown angle, and pavilion angle. The rest of the information is a modifier on top of that.
For example h&a and excellent symmetry grades are both modifiers that can lower the number calculated in the first step which was not in the original hca but was added later.

 

[Garry H (Cut Nut)]

Interesting discussion.
Almost all the changes you mention were introduced when we added Looks Like diamonds size - which I believe is an additional assistance to consumers. Therefore we need the diamonds dimensions. LLs is more accurate than spread alone because it subtracts (or rarely adds) to the diamonds apparent size.
HCA does infact calculate the girdle thickness and that was done based on the crown height (table size and crown angle) and pavilion depth (angle and originally minus culet height but so few diamonds have a culet these days we dropped it).
Karl is correct - girdle thickness does almost zip until it gets extremely thick and then the stone will be over 2.0 anyway or the warning pops up and will not let you get a score.
Regarding the number of GIA VG's that get HCA <2.0 - I mentioned in a post last week or so that GIA seems to now accept diamonds with much shallower crown and pavilion combinations - they have clearly changed their algorithms. I think Karl found a stone less than 57.5%epth with GIA Excellent cut.
As for the broadening or ranges of proportions - I have always fought against the 34-35 40.7 to 40.8 - because this works and different proportions within the range shown on this chart have different benefits and attributes.

 

[Garry H (Cut Nut)]

This may be what your looking for:
The primary numbers driving the hca score are table%, crown angle, and pavilion angle. The rest of the information is a modifier on top of that.
For example h&a and excellent symmetry grades are both modifiers that can lower the number calculated in the first step which was not in the original hca but was added later.

Thanks Karl.
It is roughly 50% pavilion
40% crown angle
10% table size (but as diamonds get bigger tables should get smaller - not implemented in HCA)

 

[Texas Leaguer]

D:This is honestly the first time I heard that GIA very good can be AGS0. That somewhere means AGS is more lenient than GIA. The popular belief is that AGS is the most stringent when it comes to grading.

Karl and Garry (the developer of the HCA tool) have addressed your specific questions about the HCA. With regard to the question above, I can will say this:

It is rare for an AGS Ideal that has been through the ray tracer to get anything but a GIA Ex. The light performance based system is both more exacting and more stringent. However, this can happen in a couple of different ways. One way is for the GIA grader to render an opinion about meet point symmetry or issues with the girdle that impact either the symmetry or polish grade.

Another unfortunate way that some precision cut diamonds get a VG grade from GIA involves an assessment of "brillianteering". Because the GIA system cannot distinguish intentional painting to remove small leakages from techniques intended to retain weight or disguise wavy girdles and such (so called 'cheating' or swindling'), they apply a broad penalty to any diamond cut outside of a certain deviation from standard indexing. In this way they sometimes "throw the baby out with the bathwater"!

The most famous example is the EightStar super ideal. The upper girdle facets are tweaked very precisely to remove an addition 5% of leakage. However, GIA will apply a penalty for this. So, in return for the skill and effort of making a diamond with maximum brightness, the cutter gets a downgrade from GIA!

Because AGS is ray tracing a 3D model of the diamond and calculating brightness, leakage, contrast and fire, the diamond is 'seen' for the actual light performance it produces.

Top Ideal Scope image shows typical small leakages (white) in standard indexing in a precision cut AGS Ideal. Bottom images show precision cut AGS Ideal with even the minor leakages removed.

 

[Karl_K]

Another unfortunate way that some precision cut diamonds get a VG grade from GIA involves an assessment of "brillianteering". Because the GIA system cannot distinguish intentional painting to remove small leakages from techniques intended to retain weight or disguise wavy girdles and such (so called 'cheating' or swindling'), they apply a broad penalty to any diamond cut outside of a certain deviation from standard indexing.

Here is some of the disagreement I was talking about. :}
Because of the impact on scintillation I agree with GIA on these diamonds. Other experts don't.

I will see if I can find the list I made of some combos that can potentially make AGS0 but not GIA EX.

 

[John Pollard]

Like many things in gemology, the influence of crown-only painting happens on a sliding scale.

I compiled the data below using diamonds with average crown painting (ACP) from 0.5 - 6.8 degrees.



Full article here
Visible effects of painting & digging on super-ideal diamonds

The most famous example is the EightStar super ideal. The upper girdle facets are tweaked very precisely to remove an addition 5% of leakage. However, GIA will apply a penalty for this.

Eightstar tended to have ACP circa 6 degrees. They were doing it long before 2006, which was the year GIA introduced their cut grade. As it turned out, 6 degrees was beyond GIA's cutoff for EX (see the data above).

However, the 3D models of those diamonds were able to earn the values needed for Ideal 0 when ray-traced in the AGS light performance system.

Because of the impact on scintillation I agree with GIA on these diamonds. Other experts don't.

Point of order, your honor. Neither system purports to analyze scintillation. I submit to the court, therefore, that your mutual 'agreement' is merely coincidence.

(Pulling @Karl_K 's tail here, the notorious painter-hater).

I will see if I can find the list I made of some combos that can potentially make AGS0 but not GIA EX.

As published: Any RB with PA under 40.6.

 

[Texas Leaguer]

Here is some of the disagreement I was talking about. :}
Because of the impact on scintillation I agree with GIA on these diamonds. Other experts don't.

I will see if I can find the list I made of some combos that can potentially make AGS0 but not GIA EX.

Karl, I agree with you that there is something to the idea that eliminating this small leakage has some unintended consequences for scintillation. It's actually a very interesting topic. However, this was not GIA's rationale for the penalty. It is because of the limitations of a grading system that doesn't distinguish between good painting and bad painting.

It may be a matter of taste whether someone prefers more brightness and less scintillation, just as some prefer to trade off some brightness for more fire. But for GIA to grade an EightStar as anything but an Excellent cut is a clear error in my view.

 

[Karl_K]

I have an 8* story.
At the time wayyyy back when there was a ton of hype about 8* being the best so I went and looked at some in person and was totally underwhelmed.
So rather than saying all them folks are crazy and forgetting about diamonds, I set out to learn more about diamond cut with even more determination.

 

[Karl_K]

Point of order, your honor. Neither system purports to analyze scintillation. I submit to the court, therefore, that your mutual 'agreement' is merely coincidence.

(Pulling @Karl_K 's tail here, the notorious painter-hater).

Even a blind chipmunk might bite your toe on occasion.
Or was that supposed to be a squirrel.........

 

[yssie]

Even a blind chipmunk might bite your toe on occasion.
Or was that supposed to be a squirrel.........

gonna need a rabies series either way

...my contributions here are invaluable i know

 

[DiamDude]

Thank you experts for all the knowledge provided....

I have an right example here to prove that there are other factors too that should be considered to

Check light performance of the diamond that is girdle thickness and measurements of the diamond.

Below are two images for reference in which what scores predict ASET does not match to it.

The Image here is with HCA 1.6 certificate number is
2347307864 and if you check ASET image this does not match to HCA score for reference we have another image that shows more light performance compare to 1.6

In this image you will see it looks very similar to the above image however this diamond with certificate number 5333058078 has an score of 3.8 and this work totally opposite to the score compared to the first image.

As per theory knowledge by Garry Holloway any diamond that scores below 2 will have great ASET and it will have excellent light performance and they have made tool in such a way where if you have diamond with medium top thick girdle or slightly thick to medium that is 3.5 or 4.00 girdle if you try to paste certificate number on HCA then score will not pop up because on girdle with 3.5 and 4.0 diamond is unpredictable either it will have no light leakages if its been cut to ideal proportions or it can give bad ASET. Hence they have design tool with only numbers of crown and pavilion angles.

I did check all the proportions for round shape diamond and if you check any diamond which is ideal cut or excellent cut but it is more rounded compare to other measurements for an example measurements should have difference of just 0.2 or any measurement that is below difference of 0.3 that diamond is more of round and if you try to find diamond by that way then other proportions will automatically will come in place and you will hardly see any light leakages in those diamonds.

 

[Karl_K]

This is not a valid ASET image the back light is way out of spec.
Even with a valid image the diamond would have issues because of some wonk going on with the pavilion and maybe crown.
Which the HCA does not take into account.

How you can tell if an ASET image has backlight problems is using this lighter area pointed to as your reference for leakage.
Using this reference on the above image renders the image as being useless.


Edit: HCA predicts Ideal-scope image results not ASET images, the ASET scope did not exist when the HCA was developed.

 

[John Pollard]

Verifying what @Karl_K suggested.

The physics of light-behavior within a diamond are reliably consistent, so the ASET images should be close, in basic light return vs leakage (not optical precision) to the models provided on the right.

This one doesn't match.



This one is more realistic, albeit perhaps a bit aggressively backlit, as well.

 

[John Pollard]

For what it's worth, most ASET setups used by sellers these days don't use backlighting. In case it's interesting to anyone, backlit vs non-backlit info can be found on this page.

ASET Scope

ASET is like the Ideal Scope but gives more detailed information. It is best for analyzing fancy shaped diamonds.

www.pricescope.com

 

[John Pollard]

Even a blind chipmunk might bite your toe on occasion.
Or was that supposed to be a squirrel.........

Holloway Nut Adviser?

 

[LLJsmom]

Holloway Nut Adviser?

That's all I got.

 

[yssie]

As per theory knowledge by Garry Holloway any diamond that scores below 2 will have great ASET and it will have excellent light performance

Here's your misunderstanding, in a nutshell. HCA isn't guaranteeing anything.
HCA is stating that
- Diamonds that score under 2 are likely to be great choices and are worth shortlisting for further investigation.
- Diamonds that score over 2 aren't likely to be great choices, and if you're trying to sort through a huge inventory quickly, go ahead and cross them out (acknowledging that you might be tossing a baby out with the bathwater).

Also, sanity check against going full-cheeked over blood red ASET under-tables. Some like pistachios, others like walnuts. Remember that white light return and coloured light return can't coexist in one place at one time.

@DiamDude are you trade? Or d'you have a Trade badge you ought not?
If you aren't trade hit the "report concern" button to bury that badge.

 

[DiamDude]

Hi Mr John Pollard I am a big fan of yours Mr Karl I still do know if you understood what I am saying.

Mr Karl Let me know your thoughts for below ASET Lets see your Expertise on it.


I just wanted to say to Mr Gary that if you can add measurement factor and girdle factor that will actually provide great scores and also it will not loose great diamonds as even I saw some diamonds with great ASET but HCA score was 5.

 

[Karl_K]

Not a valid ASET image more than likely.
To have 2 mains showing black which means shallow and one showing red which means deep and the rest blue is not something that is expected.
If it is a representative image the diamond has a ton of twist in the pavilion and someone may have modified the ASET scope..

 

[Karl_K]

great ASET but HCA score was 5.

There are many combinations that can have good looking ASET image but not get hca under 2, ags0 or even sometimes gia EX cut grades.