GIA Excellent Cut but 5.5 HCA? What gives?

[ericmits]

Hello,

I'm considering a few diamond options for an engagement ring for my gf. I have a GIA report for one that looks great in person. But when running the measurements through the HCA calculator, it got a score of 5.5. Can someone explain how this is possible?

1.36 ct, G VS2
Cut Grade: Excellent
Polish: Excellent
Symmetry: Excellent

Depth: 62.8%
Table: 57%
Crown Angle: 36.0
Pavilion Angle: 41.0
Culet: 0

girdle: medium-slightly thick (faceted) 3.5%

From my research, it looks like the crown angle seems a bit on the higher end, but then why does the stone achieve an Excellent cut grade? Who's in the wrong on this one?

 

[diamondseeker2006]

GIA Excellent is much broader than AGS Ideal so some stones are going to be better than others. That stone is too deep. What is the diameter? The other issue probably is the combination of the crown angle with the pavilion angle.

 

[Rockdiamond]

A lot of people here love to use HCA_ and it has it's strong points.
Others have found that visual aspects in real life are sometimes contrary to HCA scores.
That is to say, others have had your experience- finding a great looking GIA triple EX stone that scores poorly on HCA.

You might prefer a stone that with a better HCA score- or you may not.
IN terms of value, the HCA score has no bearing- and it's entirely likely that the seller has no idea what HCA is.

I agree with Diamondseeker that -ostensibly- 62.8 is a bit deep.
But if the stone's diameter does not suffer from this, it may not be an issue that will bother you.
Please post the measurements and we'll have a better grip on that aspect.

 

[bagelboy]

Think of an archery target where the center and first ring could be labelled AGS. Then add a few more rings and that would be the GIA target. The GIA excellent is just much bigger and easier to hit as oppossed to the AGS zero target. I think the HCA calculator leans more toward the bullseye of both targets. Diamond cutters are expected to produce the heaviest weight possible and still get that GIA Excellent cut grade. Most buyers and sellers, cutters and lab graders do not use the HCA calculator. However, I still think it gives a person a pretty good idea of how well a diamond is cut.

 

[diamondseeker2006]

Think of an archery target where the center and first ring could be labelled AGS. Then add a few more rings and that would be the GIA target. The GIA excellent is just much bigger and easier to hit as oppossed to the AGS zero target. I think the HCA calculator leans more toward the bullseye of both targets. Diamond cutters are expected to produce the heaviest weight possible and still get that GIA Excellent cut grade. Most buyers and sellers, cutters and lab graders do not use the HCA calculator. However, I still think it gives a person a pretty good idea of how well a diamond is cut.

Great post! And the reason some of us recommend diamonds in that inner circle is because when we cannot compare a full range of GIA excellent and AGS0 stones, we cannot really know for sure what our boundaries would be. So that is why that inner circle is a safe zone for those buying by distance or those whose local jeweler doesn't have multiple ideal stones from which to choose. In fact, they may not have ANY stones from that AGS0 circle and a "good" stone may look the best of what they have to offer. This forum generally attracts people who are seeking a high quality stone, so we just try to recommend the best cut and then balance the other specs with the budget.

 

[slg47]

the OP asked who's in the wrong...the answer is no one. GIA uses different parameters for grading cut than HCA.

 

[ericmits]

Thanks for the comments, everyon! On the HCA visualization, this does appear on the outer reaches of what qualifies as an AGI excellent cut.

The measurements are 7.03 - 7.08 x 4.43 (1.36 ct)



Since we're on topic, another lovely stone I looked at scores a bit better on HCA (3.2), but not quite at AGS 0 I suppose:
1.4 ct H VS1
AGI Triple Excellent
7.17 - 7.21 4.47

Depth 62.1
Table 56
Crown 34.5
Pavilion 41.2
Culet None

Any thoughts about these proportions?

 

[ericmits]

I mean GIA, not AGI of course.

 

[diamondseeker2006]

Thanks for the comments, everyon! On the HCA visualization, this does appear on the outer reaches of what qualifies as an AGI excellent cut.

The measurements are 7.03 - 7.08 x 4.43 (1.36 ct)



Since we're on topic, another lovely stone I looked at scores a bit better on HCA (3.2), but not quite at AGS 0 I suppose:
1.4 ct H VS1
AGI Triple Excellent
7.17 - 7.21 4.47

Depth 62.1
Table 56
Crown 34.5
Pavilion 41.2
Culet None

Any thoughts about these proportions?

That one has a problem with the pavilion angle. The pavilion angle should not be over 41.0 with that crown angle. Look for 34-35 crown angle and 40.6-41.0 on the pav. angle. 54-58 table, 60-62.3 depth, girdle thin, medium, slightly thick. Those will get you in the ideal cut range.

 

[WishfulThinkingOne]

HCA knocks this combination due to the light leakage at the edge of the table and loss of brilliance associated with this combination.
It is primarily pointing out their are better combinations of GIA Ex stones to consider over this one.

The worst problem areas are the ring of white where no light will be returned. The gray areas will be ones of reduced brilliance as well.

 

[WishfulThinkingOne]

Thanks for the comments, everyon! On the HCA visualization, this does appear on the outer reaches of what qualifies as an AGI excellent cut.

The measurements are 7.03 - 7.08 x 4.43 (1.36 ct)



Since we're on topic, another lovely stone I looked at scores a bit better on HCA (3.2), but not quite at AGS 0 I suppose:
1.4 ct H VS1
AGI Triple Excellent
7.17 - 7.21 4.47

Depth 62.1
Table 56
Crown 34.5
Pavilion 41.2
Culet None

Any thoughts about these proportions?

Not as bad as the first combination but the pink/white areas are potential zones of reducted brilliance as compared to red areas. AGS ideal stones would not have these areas of reduced potential brilliance.

 

[yssie]

Think of an archery target where the center and first ring could be labelled AGS. Then add a few more rings and that would be the GIA target. The GIA excellent is just much bigger and easier to hit as oppossed to the AGS zero target. I think the HCA calculator leans more toward the bullseye of both targets. Diamond cutters are expected to produce the heaviest weight possible and still get that GIA Excellent cut grade. Most buyers and sellers, cutters and lab graders do not use the HCA calculator. However, I still think it gives a person a pretty good idea of how well a diamond is cut.

True... but there's an important point missing here. One may just *prefer* something other than what everyone else prefers, including the people that created the HCA and AGS (and heck even GIA) cut grade systems! If I was in the market for a 2ct for example THIS would be my first choice, above everything else - and horror of horrors it's not even GIA EX by the proportions: http://www.jamesallen.com/diamonds/H-VS1-Premium-Cut-Round-Diamond-1346299.asp


GIA VG/VG/VG 56/62.8/37/40.4/75, thin-slthk girdle, no culet by the GIA report, HCA 2.3


I disagree strongly with DS and wishfulthinking on the second point also -

1. When you're talking about tools that require detail to the 0.1degree you have to remember that such precision is fool's gold, since the scanners AGS uses for example are only accurate to +/- 0.1degree. And GIA rounds pav up to nearest 0.2deg anyway - so nitpicking over 41 vs. 41.2 as listed on the report is a pointless endeavour. I'd file that 34.5/41.2 in my "to get more information about" folder no question.

2. Those DC outputs are assuming perfect optical symmetry, which from just the numbers provided we know nothing about - and this is in addition to the averaging and rounding we know about but can't quantify without more info, again.

I'll post some examples in a minute.. (edited to add info)

 

[yssie]

Okay here we go -

1. A quartet of GIA-EX HCA "passes" that rather frighten me: (from L-R)

http://www.jamesallen.com/diamonds/D-VVS1-Ideal-Cut-Round-Diamond-1427964.asp (HCA 1.2) 56/34/41/80 GIA-EX
http://www.jamesallen.com/diamonds/G-SI1-Ideal-Cut-Round-Diamond-1397149.asp (HCA 1.8 ) 57/35/40.8/80 GIA-EX
http://www.jamesallen.com/diamonds/H-VS1-Premium-Cut-Round-Diamond-1437688.asp (HCA 2.1) 60/34/41/85 GIA-EX
http://www.jamesallen.com/diamonds/K-SI1-Ideal-Cut-Round-Diamond-1383091.asp (HCA 0.9) 58/33.5/40.8/80 GIA-EX



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2. AGS0 HCA-4.8 (and I wouldn't want it): http://www.jamesallen.com/diamonds/I-SI2-Ideal-Cut-Round-Diamond-1355422.asp 58.1/34.8/41.3/79



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3. GIA-EX HCA-4.6 that I'd recommend having pulled for further investigation in a heartbeat: http://www.jamesallen.com/diamonds/J-VVS1-Ideal-Cut-Round-Diamond-1384122.asp 58/34/41.4/80



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4. An AGS0 that HCA is merciless on (4.2) that I don't see anything at all wrong with other than not being to my personal tastes: http://www.jamesallen.com/diamonds/E-VS1-Ideal-Cut-Round-Diamond-1334677.asp 61.5/34/41.2/78



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5. Numbers tell you nothing about optical symmetry: http://www.jamesallen.com/diamonds/E-VVS2-Premium-Cut-Round-Diamond-1424821.asp 61/33/41.2/85

 

[klp834]

Wow, GREAT discussion.

 

[Karl_K]

Okay here we go -

1. A quartet of GIA-EX HCA "passes" that rather frighten me: (from L-R)

http://www.jamesallen.com/diamonds/D-VVS1-Ideal-Cut-Round-Diamond-1427964.asp (HCA 1.2) 56/34/41/80 GIA-EX
http://www.jamesallen.com/diamonds/G-SI1-Ideal-Cut-Round-Diamond-1397149.asp (HCA 1.8 ) 57/35/40.8/80 GIA-EX
http://www.jamesallen.com/diamonds/H-VS1-Premium-Cut-Round-Diamond-1437688.asp (HCA 2.1) 60/34/41/85 GIA-EX
http://www.jamesallen.com/diamonds/K-SI1-Ideal-Cut-Round-Diamond-1383091.asp (HCA 0.9) 58/33.5/40.8/80 GIA-EX



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I would considering buying the first 3.
The 4th for a pendant in a heartbeat but maybe on a ring without more info.
If you saw IS/ASET or well shot regular images you would be very surprised.

edit: that is only taking cut into account not value, they aren't top end stones.

 

[Karl_K]

Okay here we go -



4. An AGS0 that HCA is merciless on (4.2) that I don't see anything at all wrong with other than not being to my personal tastes: http://www.jamesallen.com/diamonds/E-VS1-Ideal-Cut-Round-Diamond-1334677.asp 61.5/34/41.2/78

It is not my cup of tea and not one I would really recommend buying without seeing it in person but the HCA is overly harsh. You are right about that.

 

[Karl_K]

True... but there's an important point missing here. One may just *prefer* something other than what everyone else prefers, including the people that created the HCA and AGS (and heck even GIA) cut grade systems! If I was in the market for a 2ct for example THIS would be my first choice, above everything else - and horror of horrors it's not even GIA EX by the proportions: http://www.jamesallen.com/diamonds/H-VS1-Premium-Cut-Round-Diamond-1346299.asp


GIA VG/VG/VG 56/62.8/37/40.4/75, thin-slthk girdle, no culet by the GIA report, HCA 2.3

I like that one a lot :}
Know someone who owns its larger twin and I helped them pick it out locally.
HCA likes it and Garry would like it.

 

[yssie]

I like that one a lot :}
Know someone who owns its larger twin and I helped them pick it out locally.
HCA likes it and Garry would like it.

The lucky creature! It must be incredible in-person! I'd guess AGS would like it too..

I would considering buying the first 3.
The 4th for a pendant in a heartbeat but maybe on a ring without more info.
If you saw IS/ASET or well shot regular images you would be very surprised.

edit: that is only taking cut into account not value, they aren't top end stones.

I worded that poorly I should've said *I* wouldn't like them personally, I think too many inconsistencies to be due to photography, but there are definitely pretty stones w/ lower optical symmetry and people who actively seek them! But, esp after seeing the hoards of arrows closeups here on PS, they're good examples of why if you're looking for something specific you need to see the stone, or at least some good photos/video, and not just assume you know what you're getting (or not getting!) by the GIA report, HCA, whatever other number tools. Yes - you're right about #2 possibly just being the photo.

 

[elle_chris]

I own a 34/41/56/80 and it does not look like the one you posted, Yssie. bleh
In fact, it's one of my favorite combos (not because i own it). I think it's just slightly brighter than the preferred numbers for ideal cuts.

 

[Karl_K]

I think too many inconsistencies to be due to photography, but there are definitely pretty stones w/ lower optical symmetry and people who actively seek them!

I could make any RB no matter how well cut look like that in a photo.
Would they look like that in the real world on the finger... no.
Sure there are some optical symmetry deviations but if the price was right I would consider them.
Not everyone can afford a super-ideal and some may not want them.