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Cut and HCA

smelvinwrong

Rough_Rock
Joined
Oct 23, 2014
Messages
36
First time newb here.. but was curious about something.

Does a diamond's cut still matter if they both rank high on the HCA tool? Say if one is graded as "Good" with a 1.2 HCA and another as "Excellent" with 1.6 HCA on Blue Nile.

Since the "Good" cut has a better overall visual performance, should I be giving more consideration to the HCA score or to the cut grade?
 

canvasback

Shiny_Rock
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Aug 18, 2014
Messages
177
We only care that the HCA is below 2. A 1.4 is not better than a 2. HCA is only a rejection tool... Meaning, if RB scores below a 2, it might be worth further examination.



Cut always, always, always matters. Forget HCA once you see it is below 2.
 

Gypsy

Super_Ideal_Rock
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Aug 8, 2005
Messages
40,225
HCA is pass fail. It is also a rejection tool. Not a selection tool.

The entire purpose of faceting a diamond is to reflect light.
How well or how poorly a diamond does this determines how beautiful it is.
How well a diamond performs is determined by the angles and cutting. This is why we say cut is king.
No other factor: not color, not clarity has as much of an impact on the appearance of a diamond as its cut. An ideal H will out white a poorly cut F. And GIA Ex is not enough. And you must stick to GIA and AGS only. EGL is a bad option: [URL='https://www.pricescope.com/community/threads/egl-certification-are-any-of-them-ok.142863/']https://www.pricescope.com/community/threads/egl-certification-are-any-of-them-ok.142863/[/URL]
So how to we ensure that we have the right angles and cutting to get the light performance we want?
https://www.pricescope.com/wiki/diamonds/diamond-cut
Well one method is to start with a GIA Ex, and then apply the HCA to it. YOU DO NOT USE HCA for AGS0 stones.
https://www.pricescope.com/wiki/diamonds/holloway-cut-advisor
The HCA is a rejection tool. Not a selection tool. It uses 4 data points to make a rudimentary call on how the diamond may perform.
If the diamond passes then you know that you are in the right zone in terms of angles for light performance. Under 2 is a pass. Under 2.5-2.1 is a maybe. 2.6 and over is a no. No score 2 and under is better than any other.
Is that enough? Not really.
So what you need is a way to check actual light performance of your actual stone.
That's what an idealscope image does. https://www.pricescope.com/wiki/diamonds/firescope-idealscope
It shows you how and wear your diamond is reflecting light, how well it is going at it, and where you are losing light return. That is why you won't see us recommending Blue Nile, as they do not provide idealscope images for their diamonds. BGD, James Allen, GOG, HPD, ERD and WF do.

The Idealscope is the 'selection tool'. Not the HCA.
So yes, with a GIA stone you need the idealscope images. Or you can buy an idealscope yourself and take it in to the jeweler you are working with to check the stones yourself. Or if you have a good return policy (full refund minimum 7 days) then you can buy the idealscope, buy the stone, and do it at home.


Now if you want to skip all that... stick to AGS0 stones and then all you have to do is pick color and clarity and you know you have a great performing diamond. Because AGS has already done the checking for you. That's why they trade at a premium.
 

drk14

Brilliant_Rock
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Jun 25, 2014
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1,061
"RB" means "round brilliant" (i.e., a round diamond). You will also see the abbreviation "MRB" ("modern round brilliant").

Rephrasing OP's question, if a diamond passes the HCA<2.0 test, does it matter if it only has a GIA cut grade of "Good"? If a hypothetical MRB with "Good" cut has HCA=1.9, and another hypothetical MRB has a cut grade of "Excellent", but HCA=2.1, which is better? I've pulled these numbers out of a hat, but the question is: at what point does the GIA cut grade trump the HCA (or vice versa)?
 

smelvinwrong

Rough_Rock
Joined
Oct 23, 2014
Messages
36
Thanks for all the replies! Definitely helps to clear up that the cut has the priority. I'll look for an ideal/excellent cut first, then eliminate those who do not pass the <2.0 HCA test. Thanks!
 

Snowdrop13

Ideal_Rock
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drk14, I'd like to know the answer to that question too, having seen at least 1 GIA "very good cut" score less than 2.
 

Diamondbug

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Jul 23, 2009
Messages
949
I think GIA cut grade takes into acct the polish and symmetry. For example this GOG stone that I was eyeing but I was too slow pulling the trigger. It scores very well on HCA with a 1.3. Excellent in fire, light return and scintillation and very good in spread but it only received a GIA very good cut grade. I think it is because of the good polish and very good symmetry. I personally thinks it is an awesome looking stone even though GIA only gave it a Very Good cut grade.

http://www.goodoldgold.com/diamond/12333/

Thats what I think...hope to hear what the experts says about this.
 

DBH

Rough_Rock
Joined
Jun 20, 2014
Messages
4
Re: Cut and HCA
Post by Diamondbug » 29 Oct 2014 12:35
I think GIA cut grade takes into acct the polish and symmetry. For example this GOG stone that I was eyeing but I was too slow pulling the trigger. It scores very well on HCA with a 1.3. Excellent in fire, light return and scintillation and very good in spread but it only received a GIA very good cut grade. I think it is because of the good polish and very good symmetry. I personally thinks it is an awesome looking stone even though GIA only gave it a Very Good cut grade.

http://www.goodoldgold.com/diamond/12333/

Thats what I think...hope to hear what the experts says about this.


I'm definitely no expert, but I think why the particular stone in your example only received a Very Good cut grade from GIA is it's poor length and width dimensions. On most Excellent cut stones, there is only a few 1/100ths of a millimeter discrepancy from being truly round. Unless there was a typo, this stone was listed as having a width of 8.04 mm (8.05 on the GIA report) and a length of 8.18 mm..... a rather HUGE spread. The other listed dimensions for that stone appear to be Excellent or Ideal ratings.
 

Texas Leaguer

Ideal_Rock
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Diamondbug|1414607733|3774554 said:
I think GIA cut grade takes into acct the polish and symmetry. For example this GOG stone that I was eyeing but I was too slow pulling the trigger. It scores very well on HCA with a 1.3. Excellent in fire, light return and scintillation and very good in spread but it only received a GIA very good cut grade. I think it is because of the good polish and very good symmetry. I personally thinks it is an awesome looking stone even though GIA only gave it a Very Good cut grade.

http://www.goodoldgold.com/diamond/12333/

Thats what I think...hope to hear what the experts says about this.
Yes, the GIA overall cut grade cannot be more than one grade higher than either polish or symmetry no matter how well proportioned. Therefore, if a diamond has a grade of Good for either polish or symmetry, the highest overall cut grade that can be assigned is Very Good (irrespective of how good the HCA score might be).
 

drk14

Brilliant_Rock
Joined
Jun 25, 2014
Messages
1,061
Texas Leaguer|1414701692|3775135 said:
Yes, the GIA overall cut grade cannot be more than one grade higher than either polish or symmetry no matter how well proportioned. Therefore, if a diamond has a grade of Good for either polish or symmetry, the highest overall cut grade that can be assigned is Very Good (irrespective of how good the HCA score might be).
Bryan,
Thanks for the insight. Nonetheless, the question remains for me: at what point does the GIA cut grade trump the HCA score (or vice versa)? For example, I've seen MRBs that had a GIA cut grade of "Good" (not even VG), but HCA=0.5. What kind of light performance can be expected?

The PS conventional wisdom* seems to be that if the GIA cut grade is not Excellent, it is not even worth bothering with the HCA (implying that light performance of diamond rated VG for any reason, including polish or symmetry, will have inadequate light performance even if the diamond proportions result in HCA<2). Is this accurate, or is there a more nuanced analysis of the interplay between cut grade and HCA score?

*I do understand that to efficiently help newcomers, sometimes the "rules" of diamond selection have to be simplified. But I'm still interested in the answer to the questions posed in this thread. Thank you!
 

Texas Leaguer

Ideal_Rock
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drk14|1414707507|3775174 said:
Texas Leaguer|1414701692|3775135 said:
Yes, the GIA overall cut grade cannot be more than one grade higher than either polish or symmetry no matter how well proportioned. Therefore, if a diamond has a grade of Good for either polish or symmetry, the highest overall cut grade that can be assigned is Very Good (irrespective of how good the HCA score might be).
Bryan,
Thanks for the insight. Nonetheless, the question remains for me: at what point does the GIA cut grade trump the HCA score (or vice versa)? For example, I've seen MRBs that had a GIA cut grade of "Good" (not even VG), but HCA=0.5. What kind of light performance can be expected?

The PS conventional wisdom* seems to be that if the GIA cut grade is not Excellent, it is not even worth bothering with the HCA (implying that light performance of diamond rated VG for any reason, including polish or symmetry, will have inadequate light performance even if the diamond proportions result in HCA<2). Is this accurate, or is there a more nuanced analysis of the interplay between cut grade and HCA score?

*I do understand that to efficiently help newcomers, sometimes the "rules" of diamond selection have to be simplified. But I'm still interested in the answer to the questions posed in this thread. Thank you!
It's a good question drk. I think it's a sequence or a filtering process that the PS community recommends for GIA stones. First filter for Excellent, then filter for HCA of 2 or under. If you have plenty of candidates there is no particular reason to widen the search. Between the two filters you have removed alot of potential problems for light performance and you can focus on the other C's and study ASET, IS and H&A images ( if available) to further narrow the contenders.

However, it is quite possible to have a great stone that falls to Very Good because of a polish or symmetry fault. At that point you really need to have eye's on the prize, and preferably trained eyes. For example a polish problem could effect light performance in a subtle but significant way. Or it might be isolated to a few facets and have negligible effect.

I hope that helps a little.
 

smelvinwrong

Rough_Rock
Joined
Oct 23, 2014
Messages
36
Very interesting, thanks for all the additional insight and replies.

I guess I might as well give you some more specifics and see if you could help narrow down my search. First my primary goal was to get 1 carat. Then obviously the cut and HCA <2 comes next. At least an SI1 eye clean and preferably VS1/2 clarity. Color is last on my list, but no more than "L" ranking which I figure if it also has some blue fluro would work to make it look brighter. Aaandd all of this to come in around $4k. I could go up just a bit, but I'd like to get the whole ring/setting done under $5k.

Am I way off base to expect a 1 carat EX/VG round diamond for 4k?
 

drk14

Brilliant_Rock
Joined
Jun 25, 2014
Messages
1,061
Texas Leaguer|1414708291|3775176 said:
It's a good question drk. I think it's a sequence or a filtering process that the PS community recommends for GIA stones. First filter for Excellent, then filter for HCA of 2 or under. If you have plenty of candidates there is no particular reason to widen the search.
Thanks, Bryan, for this helpful information. You are right, from a practical standpoint, the need to contemplate these issues would come into play only if the number of candidates is small (e.g., if the budget is small for the desired diamond specifications).
And it is an interesting discussion from a purely theoretical standpoint, in any case...

So, assuming HCA<2, are there specific GIA grades that would be sufficiently alarming to disqualify a diamond right off the bat (without even looking at reflector images)? Conversely, are there specific grade combinations in which Cut<Excellent that are worth a second look if HCA<2?

For example, if Cut=VG or G, with Pol=X, Sym=X, this implies the Cut grade was not due to Pol/Sym issues. If such a diamond has HCA<2, what is one to conclude? Are there any other combinations of Cut/Pol/Sym grades that are a priori either favorable or deal-breakers (again assuming HCA<2)?

So, basically what I'm looking for is, are there any reasonable ways to expand the prevailing heuristic to "first consider only GIA XXX, then select for HCA<2" when shopping for GIA stones?
 

drk14

Brilliant_Rock
Joined
Jun 25, 2014
Messages
1,061
smelvinwrong|1414709972|3775190 said:
Very interesting, thanks for all the additional insight and replies.

I guess I might as well give you some more specifics and see if you could help narrow down my search. First my primary goal was to get 1 carat. Then obviously the cut and HCA <2 comes next. At least an SI1 eye clean and preferably VS1/2 clarity. Color is last on my list, but no more than "L" ranking which I figure if it also has some blue fluro would work to make it look brighter. Aaandd all of this to come in around $4k. I could go up just a bit, but I'd like to get the whole ring/setting done under $5k.

Am I way off base to expect a 1 carat EX/VG round diamond for 4k?


Here's a 1.01ct-K-VS2:
http://www.b2cjewels.com/dd-6250315-1.01-carat-Round-diamond-K-color-VS2-Clarity.aspx?sku=6250315
and a 1.00ct-I-SI1:
http://www.b2cjewels.com/dd-5296835-1.00-carat-Round-diamond-I-color-SI1-Clarity.aspx?sku=5296835

You might need to go down to eye-clean SI2 to find a larger number of potential candidates.


FYI, if you don't get other responses, try re-posting your search criteria in a new thread. Your request might not get seen here, at the bottom of a thread about cut grading and HCA...
 

MelisendeDiamonds

Shiny_Rock
Trade
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Jul 2, 2014
Messages
234
Texas Leaguer|1414708291|3775176 said:
However, it is quite possible to have a great stone that falls to Very Good because of a polish or symmetry fault. At that point you really need to have eye's on the prize, and preferably trained eyes. For example a polish problem could effect light performance in a subtle but significant way. Or it might be isolated to a few facets and have negligible effect.

+1 Good explanation. Site unseen over the internet I'd tend to recommend safer choices that are GIA Excellent.
 

Karl_K

Super_Ideal_Rock
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drk14|1414707507|3775174 said:
Thanks for the insight. Nonetheless, the question remains for me: at what point does the GIA cut grade trump the HCA score (or vice versa)? For example, I've seen MRBs that had a GIA cut grade of "Good" (not even VG), but HCA=0.5. What kind of light performance can be expected?
In an earring or pendant possibly stunning in a ring so-so if the grade was not based on finish and the angles are right.
Its a quirk or feature depending on who you ask of the hca that it considers diamonds for all uses where ags0 and giaEX are close range ring centric.
 

Texas Leaguer

Ideal_Rock
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drk14|1414715995|3775231 said:
Texas Leaguer|1414708291|3775176 said:
It's a good question drk. I think it's a sequence or a filtering process that the PS community recommends for GIA stones. First filter for Excellent, then filter for HCA of 2 or under. If you have plenty of candidates there is no particular reason to widen the search.
Thanks, Bryan, for this helpful information. You are right, from a practical standpoint, the need to contemplate these issues would come into play only if the number of candidates is small (e.g., if the budget is small for the desired diamond specifications).
And it is an interesting discussion from a purely theoretical standpoint, in any case...

So, assuming HCA<2, are there specific GIA grades that would be sufficiently alarming to disqualify a diamond right off the bat (without even looking at reflector images)? Conversely, are there specific grade combinations in which Cut<Excellent that are worth a second look if HCA<2?

For example, if Cut=VG or G, with Pol=X, Sym=X, this implies the Cut grade was not due to Pol/Sym issues. If such a diamond has HCA<2, what is one to conclude? Are there any other combinations of Cut/Pol/Sym grades that are a priori either favorable or deal-breakers (again assuming HCA<2)?

So, basically what I'm looking for is, are there any reasonable ways to expand the prevailing heuristic to "first consider only GIA XXX, then select for HCA<2" when shopping for GIA stones?
I guess I would say that if either polish or symmetry is down to fair or poor it is a deal breaker. If one or the other is only good, it warrants consideration and more detailed analysis. But only if other options aren't available or are significantly more expensive. I would be more concerned with low polish grade, because that will not show in light performance imaging. And it really takes a trained eye to determine whether it will present a significant problem for light performance. Some top experts think the level of polish (facet flatness) has a dramatic effect on certain aspects of light performance.
 

drk14

Brilliant_Rock
Joined
Jun 25, 2014
Messages
1,061
Thanks Karl and Bryan...very helpful/interesting!

I was confused by the following, though:
Texas Leaguer|1414771003|3775457 said:
I would be more concerned with low polish grade, because that will not show in light performance imaging. And it really takes a trained eye to determine whether it will present a significant problem for light performance. Some top experts think the level of polish (facet flatness) has a dramatic effect on certain aspects of light performance.
If the effect is dramatic, why does it take a trained eye to detect it? Or did you mean it takes a trained eye to analyze the diamond specifications and reflector imagery to make an accurate prediction about whether the "dramatic effect" will be present?
 

Texas Leaguer

Ideal_Rock
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drk14|1414793560|3775688 said:
Thanks Karl and Bryan...very helpful/interesting!

I was confused by the following, though:
Texas Leaguer|1414771003|3775457 said:
I would be more concerned with low polish grade, because that will not show in light performance imaging. And it really takes a trained eye to determine whether it will present a significant problem for light performance. Some top experts think the level of polish (facet flatness) has a dramatic effect on certain aspects of light performance.
If the effect is dramatic, why does it take a trained eye to detect it? Or did you mean it takes a trained eye to analyze the diamond specifications and reflector imagery to make an accurate prediction about whether the "dramatic effect" will be present?
My point is that it is a non-obvious aspect that can have disproportionate impacts on performance. Maybe Karl can speak more to the facet flatness aspect, which is related to polish. Serg has mentioned it in his research and I seem to recall that he has made a point of it's importance to fire and possibly to other aspects of performance.

Polish problems compromise the diamond/air boundary leading to unintended consequences with reflection and refraction of light rays which can diminish performance. A similar effect takes place when a diamond gets dirty, although that can be reversed.
 
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