GIA Cut Rated Good, With HCA of 1?

[Andelain]

I saw a stone with a GIA grade of only good, yet it scores 1 on the HCA. The crown and pav angles are 35/40.6, but the depth is 63.5 and it has a very thick girdle. What kind of performance is this one likely to have? How much light is lost because of the depth when the angles are good. Besides the obvious smaller diameter, that is.

TIA

 

[diamondseeker2006]

I only use the HCA to narrow down the GIA Excellent category. I am not sure the HCA matters on lower than excellent because we know very good and good have problems! Obviously I have no idea of the answer to your question. One of the guys like John P or Karl needs to come and show you some graphics!

 

[flyingpig]

http://www.diamondcut.gia.edu/pdfs/estimating_cut_grade_chart_lowres.pdf

I think this may answer your question.
Please see the first stone in the very good column.

Table size: 53%
Crown angle: 34.5°
Pavilion angle: 40.8°
Star length: 55%
Lower half length: 80%
Girdle thickness: 4.5%
(Slightly Thick to Thick)
Culet size: Medium

All parameters are within the
Excellent range except total
depth, girdle thickness, and
culet size. The face-up appearance
may be as good as
those in the Excellent range,
but the diamond has too
much weight for its diameter

 

[Karl_K]

hca assumes a med girdle and also at those angles and depth the individual facet angles are likely not accurately reflected in the rounded then averaged then grossly rounded gia numbers.
How it looks other than small is anyones guess based on the data presented.

 

[Karl_K]

cant produce pretty diagrams right now because my computer is still down.
The question of how a huge girdle between a crown and pavilion of good angles and make perform is a complex one.
Discounting face up size, many times face up performance would not be hugely impacted but under tilt there may be eye noticeable degradations in performance.
It depends on the specific angles involved.

 

[John P]

I saw a stone with a GIA grade of only good, yet it scores 1 on the HCA. The crown and pav angles are 35/40.6, but the depth is 63.5 and it has a very thick girdle. What kind of performance is this one likely to have?

Table size? Culet?

Inquiring minds want to know.

 

[Andelain]

Table size? Culet?

Inquiring minds want to know.

Table is 54, culet is pointed.

 

[Andelain]

http://www.diamondcut.gia.edu/pdfs/estimating_cut_grade_chart_lowres.pdf

I think this may answer your question.
Please see the first stone in the very good column.

Table size: 53%
Crown angle: 34.5°
Pavilion angle: 40.8°
Star length: 55%
Lower half length: 80%
Girdle thickness: 4.5%
(Slightly Thick to Thick)
Culet size: Medium

All parameters are within the
Excellent range except total
depth, girdle thickness, and
culet size. The face-up appearance
may be as good as
those in the Excellent range,
but the diamond has too
much weight for its diameter

I can't get your pdf to load here or at work.

Along with the angles I gave above, the LGF's are 85% on this stone. As I said above, it does lose diameter. I did notice it's price is more in line with face up size than weight, but that's irrelevant if it's not a pretty stone.

 

[Andelain]

cant produce pretty diagrams right now because my computer is still down.
The question of how a huge girdle between a crown and pavilion of good angles and make perform is a complex one.
Discounting face up size, many times face up performance would not be hugely impacted but under tilt there may be eye noticeable degradations in performance.
It depends on the specific angles involved.

And there lies who whole question. Looking at those numbers with a normal girdle, I'd figure the stone to be a winner. Add in the extra real estate, and here I am asking you experts.

 

[John P]

First of all: The HCA score of 1.1 is a prediction of light return. In this case it's correct. This diamond has great angles for robust light return. However the steep configuration and depth would make it face-up 10% smaller than it should for the carat weight... So if one were to purchase a 1.00ct in this configuration they should be aware that it's only spreading like other well-cut 0.90ct diamonds. The HCA does note that spread is "Good" (and we know "Good" doesn't necessarily mean great in the diamond biz).

Now for the nano-tech interesting stuff...only nerds need read on

The basics are 54T 63.5D 40.6PA 35.0CA VTK None: Graded GIA Good.

In theory, at those proportions (non-rounded) the girdle thickness should not run to "Very Thick." At most it should be STK. And the overall GIA cut-grade should be Very-Good.


However, we know GIA averages multiple measurements, and further rounds them.

With that said, the outlier here is girdle-thickness. "Very thick" is a strong indictment. So I pushed the rounded-numbers to the logical raw-limits which would increase girdle thickness (Rounded 54T becomes 54.4, Rounded 35.0 CA becomes 34.76, Rounded 40.6 PA becomes 40.5)... And while these increase girdle thickness by .5% - and explains the overall cut grade of "Good" - it still doesn't explain the GIA girdle judgment of Very Thick.


Curious. Curious.

What's the answer? There may be some small, undisclosed natural on the girdle running to VTK. It may be a result of scan-error. The depth figure may be faulty due to detritus on the scanning stage or poor calibration. In the spirit of the season, maybe it's channeling the dark side of the depth-force.

Either way, it's fun to work these puzzles... In this case the HCA number is correct but the spread will be 10% lower than it should be and there may be ghosts or naughty droids in the scanner.

Edited to add: After reading Andelain's further info I recalculated the above with 85% lower halves. It doesn't alter the basic grades.

 

[Andelain]

...... There may be some small, undisclosed natural on the girdle running to VTK.........[/color]

This had passed through my mind, the stone is SI-1. The grademakers are a crystal and an indented natural.

 

[John P]

...... There may be some small, undisclosed natural on the girdle running to VTK.........[/color]

This had passed through my mind, the stone is SI-1. The grademakers are a crystal and an indented natural.

As I know you know, an indented natural wouldn't be the culprit. Do you have the GIA number and the report date?

 

[Andelain]

Either way, it's fun to work these puzzles... In this case the HCA number is correct but the spread will be 10% lower than it should be and there may be ghosts or naughty droids in the scanner.

Yep, the spread is just about that of an 81-82 point stone, but it's a 90. Only reason it has me so interested is it could be a great match for an 82 that I already have, and the price may be good. That is, if the stone is going to perform like the HCA suggests.

 

[Andelain]

...... There may be some small, undisclosed natural on the girdle running to VTK.........[/color]

This had passed through my mind, the stone is SI-1. The grademakers are a crystal and an indented natural.

As I know you know, an indented natural wouldn't be the culprit. Do you have the GIA number and the report date?

I didn't know that. The report # is 16893030, dated Feb 28, 2008.

 

[John P]

...... There may be some small, undisclosed natural on the girdle running to VTK.........[/color]

This had passed through my mind, the stone is SI-1. The grademakers are a crystal and an indented natural.

As I know you know, an indented natural wouldn't be the culprit. Do you have the GIA number and the report date?

I didn't know that. The report # is 16893030, dated Feb 28, 2008.

Thanks. The report is too old for a PDF to be included. But you've reported the particulars 100% correctly.

Since you're considering this for yourself my white-knight instinct is coming into play... Candidly, you need more information on the cut-consistency. GIA's "symmetry" assessment involves 2D facet meet-points only. It says nothing about what's inside those averaged numbers. In practical terms this could have been difficult crystal where one side of the diamond did not run well on the wheel and the necessary-deviations from the averaged-angles could be extreme. If those deviations resulted in an isolated area of the girdle that ran up to VTK (where the average is nominally STK) that would explain the report descriptions and grade.

The ASET simulations I posted above presume perfect 3D precision, which is impossible in real-life and unlikely in this scenario. The actual light return footprint will be significantly different in a case like that posed above; where VTK was needed due to a difficult rough crystal.

Bottom line: GIA's averaged and rounded report numbers have left some unanswered questions. A reflector image like Ideal-Scope or ASET would be extremely helpful in understanding what's behind the curtains here.

 

[Andelain]

That definitely makes sense since the numbers don't support the VTK. Of course I didn't know that part until you did you magic for me. What do you also think of Karl's suggestion that light performance from an angle would be worse than normal for a stone with those angles?

 

[John P]

That definitely makes sense since the numbers don't support the VTK. Of course I didn't know that part until you did you magic for me. What do you also think of Karl's suggestion that light performance from an angle would be worse than normal for a stone with those angles?

* If we're imagining consistent cut (like the ASETs I posted) but a full, VTK girdle then I agree with Karl.
* If we're imagining consistent cut with STK girdle but one undisclosed natural causing VTK then it's no problem.
* If we're imagining one that didn't run well on the wheel with inconsistency causing VTK then nothing can save it.

The problem is...we're imagining. I wish practical images accompanied all grading reports.

 

[Andelain]

That definitely makes sense since the numbers don't support the VTK. Of course I didn't know that part until you did you magic for me. What do you also think of Karl's suggestion that light performance from an angle would be worse than normal for a stone with those angles?

* If we're imagining consistent cut (like the ASETs I posted) but a full, VTK girdle then I agree with Karl.
* If we're imagining consistent cut with STK girdle but one undisclosed natural causing VTK then it's no problem.
* If we're imagining one that didn't run well on the wheel with inconsistency causing VTK then nothing can save it.

The problem is...we're imagining. I wish practical images accompanied all grading reports.

I agree, with they were too. I've taken changes on stones and bought with only numbers, but only 3X or AGS0's. The chances of me actually getting this stone are almost nil at this point, but this thread has been very educational. I really appreciate the time you've taken here to share your knowledge.

I do have one more question, why wouldn't the indented natural that is disclosed have any effect on the girdle thickness rating?

 

[John P]

I agree, with they were too. I've taken changes on stones and bought with only numbers, but only 3X or AGS0's. The chances of me actually getting this stone are almost nil at this point, but this thread has been very educational. I really appreciate the time you've taken here to share your knowledge.

No problem. I'm happy to help...I enjoy puzzles! That said, I never want to see you or anyone on the wrong side of one.

I do have one more question, why wouldn't the indented natural that is disclosed have any effect on the girdle thickness rating?

An Indented Natural is a portion of the rough's original surface that dips below the polished surface. Without a GIA Report PDF or image, we have no idea where it is... It could be anywhere (although most INs appear on the pavilion or girdle). In any case, since it dips below the polished surface, it's subtractive. Even on the girdle it would make the spot where it exists thinner, not thicker.

In contrast, a Natural (not indented) is a portion of the rough's skin left on the polished surface by the cutter. It may be negligible or it may be additive; meaning just a raw-spot, or a protrusion adding thickness where it appears. Naturals usually appear on or near the girdle and would be identified on the grading report. That's why - if there's a natural causing the VTK grade - it's queerly undisclosed.

 

[Andelain]

No problem. I'm happy to help...I enjoy puzzles! That said, I never want to see you or anyone on the wrong side of one.

Here's a puzzle for you; Why would anyone in their right mind buy this stone?!? [URL='https://www.pricescope.com/community/threads/would-you-recommend-this-amazing-diamond.218293/']https://www.pricescope.com/community/threads/would-you-recommend-this-amazing-diamond.218293/[/URL]

I do have one more question, why wouldn't the indented natural that is disclosed have any effect on the girdle thickness rating?

An Indented Natural is a portion of the rough's original surface that dips below the polished surface. Without a GIA Report PDF or image, we have no idea where it is... It could be anywhere (although most INs appear on the pavilion or girdle). In any case, since it dips below the polished surface, it's subtractive. Even on the girdle it would make the spot where it exists thinner, not thicker.

In contrast, a Natural (not indented) is a portion of the rough's skin left on the polished surface by the cutter. It may be negligible or it may be additive; meaning just a raw-spot, or a protrusion adding thickness where it appears. Naturals usually appear on or near the girdle and would be identified on the grading report. That's why - if there's a natural causing the VTK grade - it's queerly undisclosed.

The pic of the GIA report that's posted does appear to show the indented natural on the girdle, but I can't tell more than that. I do see one thing looking at the pic of the stone though, it appears to be out of round in one area. I didn't see that until I looked closer just now, I wonder if that's where the natural is, I can't tell. Here's a link to the stone in question, http://loupetroop.com/listings/loose-stones-diamond/0-dot-90ct-gia-h-si1-round-brilliant-1-dot-1-hca;7356..

 

[John P]

The pic of the GIA report that's posted does appear to show the indented natural on the girdle, but I can't tell more than that. I do see one thing looking at the pic of the stone though, it appears to be out of round in one area. I didn't see that until I looked closer just now, I wonder if that's where the natural is, I can't tell. Here's a link to the stone in question, http://loupetroop.com/listings/loose-stones-diamond/0-dot-90ct-gia-h-si1-round-brilliant-1-dot-1-hca;7356........

That link helps a lot. I wouldn't read anything into the glamour photos; they're pretty but not useful for technical interpretations. By the way, the seller is a Pricescoper who certainly knows diamonds. You may wish to contact her directly if still interested.

1. Seeing full report data helps. It is true that 6.02-6.12 is a bit out of round. Mathematically, the total depth of 63.4% is calculated using the average of min-max spread (6.07), which is important as a critical non-rounded data point.

2. Now let's roll up the sleeves. To arrive at the (rounded) 16.5% CH this diamond's actual table % must be a bit smaller than the (rounded) 54% indicated, and the actual Crown Angle must be a bit steeper than the (rounded) 35.0 indicated. I made hypothetical-logical adjustments below.

3. Finally, to arrive at (rounded) 42.5% PD with no culet, the actual Pavilion Angle must be a bit shallower than the (rounded) 40.6 indicated. I made another adjustment below.


Of course we are limited to working from 2D averages. The actual variance from average and cut-consistency of the diamond are unaccounted-for. But, with that said, the given measurements now make sense together, leaving two areas for further inspection: Girdle thickness and carat weight.

RE Girdle thickness: This is simple math. TD 64.3% - CH+PD (rounded) = 4.4%. More importantly, TD 64.3% minus CH+PD (my hypotheticals) also = 4.4%. Cool. So we know the thickest portions of the girdle, meaning the broad "hill" positions where mains and halves meet, must reach 4.4% in at least one spot. Depending on cut-consistency, maybe in many spots.

Unfortunately, GIA does not describe the 16 hill-positions. The range they provide on the report (STK-VTK) is derived from the 16 valleys only; see graphic below left. Thus, we're left with a bit of a grey area and a missing 0.01ct.

To me, the answer involves a strategy which took the finished diamond from 0.89ct to 0.90ct.

RE carat weight: The polisher might have mildly painted the diamond when brillianteering the upper halves. That could slightly increase girdle thickness, mainly at the unreported half-facet meets (hills), but would secondarily affect the 16 valleys, which are reported. If so the painting was very slight; GIA would penalize it otherwise. Juuust enough to reach the commerically important 0.90ct mark - and tease us with a bit of a mystery.

If that's the answer it's good news in terms of aesthetics. Cut-consistency could still be robust, and such mild painting would have little or no impact on performance.

 

[ForteKitty]

Hi Andelain! Someone alerted me to this. I'm actually a little embarrassed it's being discussed here since since it isn't an ideal cut MRB. The diamond is very pretty but definitely isn't perfect, so it probably isn't the right stone for someone who is used to perfect symmetry. John's rendering looks almost spot on!

A little bit of history for this diamond: My uncle asked me to look for a diamond for his future fiance (would have been his third marriage, he uh... found her online, over seas, less than half his age. Yeah...), but it fell apart before he even proposed, so now he's asking me to sell it for him. He bought it when diamond prices were higher, so I think he's asking too much. But he might be willing to accept a return if it's someone who's been around the forums a long time, in case you want to check it out in person. I personally think he needs to lower the price as well and will work on that this weekend. Message me if you have any questions!

 

[John P]

Hi Andelain! Someone alerted me to this. I'm actually a little embarrassed it's being discussed here since since it isn't an ideal cut MRB. The diamond is very pretty but definitely isn't perfect, so it probably isn't the right stone for someone who is used to perfect symmetry. John's rendering looks almost spot on!

I was hoping you'd post, ForteKitty. I hope you'll reconsider being embarrassed. This is a great educational diamond! Where some pros would dismiss it right away due to depth/spread, inside the numbers I'm convinced it's a little fireball. In fact, depth notwithstanding, the basic proportions are candidates for AGS 0-1 performance.

A little bit of history for this diamond: My uncle asked me to look for a diamond for his future fiance (would have been his third marriage, he uh... found her online, over seas, less than half his age. Yeah...), but it fell apart before he even proposed, so now he's asking me to sell it for him. He bought it when diamond prices were higher, so I think he's asking too much. But he might be willing to accept a return if it's someone who's been around the forums a long time, in case you want to check it out in person. I personally think he needs to lower the price as well and will work on that this weekend. Message me if you have any questions!

The pricing appears to compete with 0.80ct H SI1 Medium (rather than 0.90ct) which is the appropriate strategy. I'd trumpet the fact that it's eye-clean. That sets it above thousands of "comps" which litter the bottom of internet warehouse-lists; having visible inclusions, brown/green tint, undisclosed haze or lucky-certs. Explaining the cut situation is challenging. You've listed HCA 1.1 which is great (although many people don't capisce HCA). The thing is, minus depth, the geometry jibes with both GIA EX proportions and AGS 0-1 performance. So while shoppers are comparing it to "Good" cut grades it's actually performing like an EX 0.80ct. That's challenging to get-across in an ad. The offer to accept a return could be a winner. Remember to protect yourself, as well as helping potential buyers.

 

[gm89uk]

Dear John,

The other alternative which can account for the 0.89/0.90 carat is a very small culet that wasn't picked up by the GIA grader (?possible in 0.9carat).

To mathematically work it would have to be:

Pavilion: 40.6, Crown: 35.24, Culet: 1.15%, Girdle: 4.745, Table 53.9 and an average width of 6.074 (within the error margin of measurement). With the starlength at 55 and the halflength 85% I think I calculate the carat to be 0.897.

A lower pavilion with that then takes the pavilion depth to 42% rather than 42.5%. Lowering the culet would work with a 40.5 pavilion but then the girdle would be too thick to round to 4.5.

This is all academic of course but it was a fun puzzle indeed.

 

[Andelain]

Hi Andelain! Someone alerted me to this. I'm actually a little embarrassed it's being discussed here since since it isn't an ideal cut MRB. The diamond is very pretty but definitely isn't perfect, so it probably isn't the right stone for someone who is used to perfect symmetry. John's rendering looks almost spot on!

A little bit of history for this diamond: My uncle asked me to look for a diamond for his future fiance (would have been his third marriage, he uh... found her online, over seas, less than half his age. Yeah...), but it fell apart before he even proposed, so now he's asking me to sell it for him. He bought it when diamond prices were higher, so I think he's asking too much. But he might be willing to accept a return if it's someone who's been around the forums a long time, in case you want to check it out in person. I personally think he needs to lower the price as well and will work on that this weekend. Message me if you have any questions!

Hi and Merry Christmas! It was NOT my intention to embarrass you, and I apologize that it has. I really saw it as a chance to learn something, along with having some actual interest in the stone itself. I wasn't going to spill the beans that it was yours, but this thread really took off, and the wonderful Mr Pollard really gave me some fantastic info to learn from.

BTW, I'm glad your uncle dodges that bullet!

 

[ForteKitty]

Hi and Merry Christmas! It was NOT my intention to embarrass you, and I apologize that it has. I really saw it as a chance to learn something, along with having some actual interest in the stone itself. I wasn't going to spill the beans that it was yours, but this thread really took off, and the wonderful Mr Pollard really gave me some fantastic info to learn from.

BTW, I'm glad your uncle dodges that bullet!

It's educational for me as well! Although I have to admit most of it went right over my head. My uncle sure dodged a bullet. His cat was sick and he spent a ton of money on the surgery, and this woman flipped out saying the money should have been spent on "them"... which really means "her". He ended things right then.

 

[gr8leo87]

That definitely makes sense since the numbers don't support the VTK. Of course I didn't know that part until you did you magic for me. What do you also think of Karl's suggestion that light performance from an angle would be worse than normal for a stone with those angles?

* If we're imagining consistent cut (like the ASETs I posted) but a full, VTK girdle then I agree with Karl.
* If we're imagining consistent cut with STK girdle but one undisclosed natural causing VTK then it's no problem.
* If we're imagining one that didn't run well on the wheel with inconsistency causing VTK then nothing can save it.

The problem is...we're imagining. I wish practical images accompanied all grading reports.

+1

 

[ForteKitty]

I was hoping you'd post, ForteKitty. I hope you'll reconsider being embarrassed. This is a great educational diamond! Where some pros would dismiss it right away due to depth/spread, inside the numbers I'm convinced it's a little fireball. In fact, depth notwithstanding, the basic proportions are candidates for AGS 0-1 performance.

The pricing appears to compete with 0.80ct H SI1 Medium (rather than 0.90ct) which is the appropriate strategy. I'd trumpet the fact that it's eye-clean. That sets it above thousands of "comps" which litter the bottom of internet warehouse-lists; having visible inclusions, brown/green tint, undisclosed haze or lucky-certs. Explaining the cut situation is challenging. You've listed HCA 1.1 which is great (although many people don't capisce HCA). The thing is, minus depth, the geometry jibes with both GIA EX proportions and AGS 0-1 performance. So while shoppers are comparing it to "Good" cut grades it's actually performing like an EX 0.80ct. That's challenging to get-across in an ad. The offer to accept a return could be a winner. Remember to protect yourself, as well as helping potential buyers.

Thanks, John! It is a little fireball but I had such a hard time photographing it. It was ridiculously difficult for some reason. And I wasn't really embarrassed for myself, it was more on behalf of the stone because I never thought its numbers were too great even though it's really pretty. I will definitely be careful with potential buyers.

Now that you've mentioned it, I looked closer at other SI1s and was a little shocked. Being on PS all these years and always expecting SI1s to be eye clean, I never considered that many might not be eye clean. Boy, there are some really included GIA SI1s out there! So shocking since I have several GIA SI2s that are eye clean with only tiny inclusions near the edges.

 

[Andelain]

Hi and Merry Christmas! It was NOT my intention to embarrass you, and I apologize that it has. I really saw it as a chance to learn something, along with having some actual interest in the stone itself. I wasn't going to spill the beans that it was yours, but this thread really took off, and the wonderful Mr Pollard really gave me some fantastic info to learn from.

BTW, I'm glad your uncle dodges that bullet!

It's educational for me as well! Although I have to admit most of it went right over my head. My uncle sure dodged a bullet. His cat was sick and he spent a ton of money on the surgery, and this woman flipped out saying the money should have been spent on "them"... which really means "her". He ended things right then.

I hate that kitty got sick, but I'm glad it happened when it did. Good for Uncle to take the blinders off so quickly.

BTW, is kitty OK now?