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Is HCA too restrictive?

marrduk24

Shiny_Rock
Joined
Nov 30, 2020
Messages
145
All

Being a PS member and learning about diamonds has been a great Covid hobby :)

A few things about HCA:
1. Diamond beauty is to a great deal affected by their ability to return light and Idealscope/ ASET are ultimately the most reliable tools to assess light return

2. HCA is a good elimination tool. Not all diamonds with HCA < 2 will do well in light return and conversely some diamonds with HCA > 2 may do well

One thing I wonder is if HCA is too punitive to pavilion angle. Eg crown angle 35 and pavilion angle 41 will receive HCA score 2.2.

See a file I created where I put HCA scores on CA & PA grid. I am aware that AGA 000 rating is no longer based on just two dimensional proportions and rather light return and these are old guidelines.

Ps: I have not populated the combinations that I have been filtering out
6FCC0D44-4E2D-455A-8DA5-CDE31A5EFC72.jpeg
 
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flyingpig

Ideal_Rock
Premium
Joined
Nov 7, 2015
Messages
2,806
Diamonds with HCA greater than 2.0 tend to leak light. In fact, I think @Garry H (Cut Nut) has done amazing job in fine tuning the HCA 2.0~2.4 range. It is rare to find a 35/41 without leakage and it is when CA/PA starts to get steep/deep and appear smaller face up. Therefore HCA 2.0~2.2 for this combo is just right IMO.
HCA is a bit lenient to shallow crown diamonds, but those do not generally leak light and face up bigger. So, I understand why Garry like them; he specifically says HCA takes spread into consideration.
My only problem I have with HCA is the message that a diamind with HCA less than 1.0 better for pendant. This does not applies to 34~35CA/40.6PA and often confuse buyers.
No cut assessment system is perfect. GIA and AGS have their own issues, so does HCA. But again, regarding the 2.0~2.4 range, it is tuned very well.
 
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marrduk24

Shiny_Rock
Joined
Nov 30, 2020
Messages
145
Diamonds with HCA greater than 2.0 tend to leak light. In fact, I think @Garry H (Cut Nut) has done amazing job in fine tuning the HCA 2.0~2.4 range. It is rare to find a 35/41 without leakage and it is when CA/PA starts to get steep/deep and appear smaller face up. Therefore HCA 2.0~2.2 for this combo is just right IMO.
HCA is a bit lenient to shallow crown diamonds, but those do not generally leak light and face up bigger. So, I understand why Garry like them; he specifically says HCA takes spread into consideration.
My only problem I have with HCA is the message that a diamind with HCA less than 1.0 better for pendant. This does not applies to 34~35CA/40.6PA and often confuse buyers.
No cut assessment system is perfect. GIA and AGS have their own issues, so does HCA. But again, regarding the 2.0~2.4 range, it is tuned very well.

Thanks for chiming in.
One question for you, not only have I been filtering out diamonds with HCA > 2, I have also been filtering out diamonds with CA > 35, even when HCA is under 2.
There is no good reason for that, under than that crown angle is getting further away from the perfect 34.5.

So let’s take 36/40.6 combo. I understand that this combo will have more fire & scintillation than 34/41 combo. But wondering if you filter out some crown angles or pavilion angles even when HCA < 2.
 

flyingpig

Ideal_Rock
Premium
Joined
Nov 7, 2015
Messages
2,806
Thanks for chiming in.
One question for you, not only have I been filtering out diamonds with HCA > 2, I have also been filtering out diamonds with CA > 35, even when HCA is under 2.
There is no good reason for that, under than that crown angle is getting further away from the perfect 34.5.

So let’s take 36/40.6 combo. I understand that this combo will have more fire & scintillation than 34/41 combo. But wondering if you filter out some crown angles or pavilion angles even when HCA < 2.

I would not filter out 35.5CA and 36.0CA when combined with 40.6PA. From my observation, 35.5~36.0CA/40.6PA tends to leak less than 35CA/41PA, which, again, is the reason why I believe HCA 2.2 for 35/41 is appropriate and HCA is tuned well.
Having that said 35/41 is not a bad combo. I would consider this combo, along with 35.5/40.8 (which should be in the 2.0~2.4 range), as long as price is right and there is no detrimental leakage.
 

Karl_K

Super_Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
11,128
The hca is a bit harsh with with pavilions 41 and slightly over however the minors play a hug role in if those combos work and the hca does not account for them.
GIA gross rounding of the numbers also makes it even harder to separate the great from the ok by numbers.
When it comes to combos on the edges IS or ASET images are needed.
 

marrduk24

Shiny_Rock
Joined
Nov 30, 2020
Messages
145
The hca is a bit harsh with with pavilions 41 and slightly over however the minors play a hug role in if those combos work and the hca does not account for them.
GIA gross rounding of the numbers also makes it even harder to separate the great from the ok by numbers.
When it comes to combos on the edges IS or ASET images are needed.

@Karl_K thanks for chiming in. Let me make sure I interpret that point. Let’s take 35/41 combo with 61.5 depth and 56 table. You are saying if diamond has been cut with high level of precision for upper half facets, star facets and lower half facets, such that they work in harmony to return light, the diamond could be good.
 

Karl_K

Super_Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
11,128
@Karl_K thanks for chiming in. Let me make sure I interpret that point. Let’s take 35/41 combo with 61.5 depth and 56 table. You are saying if diamond has been cut with high level of precision for upper half facets, star facets and lower half facets, such that they work in harmony to return light, the diamond could be good.

depends on if we are talking about actual angles/% or gia grossly rounded numbers.
The lower half %/angle makes the largest difference with this combo.
If they are long then little to no under table leakage if they are short their will be leakage.
This might help you understand:
 
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