Crown and Pavilion Angle and the HCA

[Thegroom]

I recently read an interesting article about the development of the HCA by Garry Holloway. It goes over how the ratings were determined for brilliance, fire, and scintillation and includes some interesting diagrams showing how various combinations of measurements would score.

http://www.diamond-cut.com.au/

Later I noticed that, if you search by cut quality on the pricscope page, that it is very common for many of the super ideal diamonds available from vendors on this site to have pavilion angles in the range of 40.8 to 41 degrees combined with crown angles of 34.5 and higher.
The HCA charts would predict a better performing diamond if you combined the 40.8 pavilion with a crown angle of 34 or less degrees, yet this is not what I see predominantly for sale on the site. Now I know the HCA isn''t the final word in diamond quality and appearance, but I was just curious if there was a disadvantage to a shallower crown angle like 33 or 34 degrees with a 40.8 or 40.9 pavilion even though the HCA predicts a lower (better) scoring diamond. Any thoughts?

 

[Mara]

I think there have been some threads on this in the past, or on similar topics.




Many of the WhiteFlash ACA stones have crown angles around 35 and/or pav angles that are around 41. I think this is what you are talking about? Their scores on HCA are higher...sometimes around 1.8-2.2 or similar...though you will find ACA's that score well on HCA with different angles at times. I don' t know why this is...but I think WhiteFlash addressed this same question at some point in the past? Or maybe it was Garry.




One thing to note....you will notice that inputting 41.1 into the HCA for an otherwise well performing stone will sometimes yield an over 2.0 scoring. Turn that 41.1 into 41 on the HCA and it will usually change, and be BETTER. Take that 41 to 40.9 and again, it will improve. So now your original 2.2 scoring 41.1 pav angle stone is scoring 1.5 TIC with 40.9.




It has been asked before if the difference between 41.1 and 41 and/or 41 and 40.9 is REALLY that different? To reap something like a 1.5 TIC vs 2.2 on the HCA...is it really that big of a deal. Who really knows?




Use the HCA as help in your search, but don't use it as gospel. It should be used like any of the other tools on the various websites, as one data point amongst hopefully many.




Good luck!

 

[dimonbob]

Use Garry's HCA in combination with his Idealscope and you know you have an eye-popping diamond. Add in near perfect hearts and arrows and it is a no brainer.

 

[Mara]

Bob since you are chiming in...why is it that many of the ACA stones have the steeper crown angles and/or pavilions? I have seen enough pictures to know that they are stunning regardless, but what is the thought process behind cutting to what some may consider slightly different proportions than the 'typical' 34.5 and/or 40.5 (e.g. crown angles over 35 and pav angles over 41)? Enquiring minds want to know!

 

[Colored Gemstone Nut]

Thats funny Mara..I was wondering the same thing....Bob?

 

[valeria101]

The test is to trace different light trajectories and see where light ends... I did this for fun but by hand takes a bit of math and patience so... I leave it now for you to enjoy! There are a lot of strange angles which avoid light leaks, but look way strange for any sane cutter to attempt.

 

[PANOS]

Hi I do not know if anyone can help me with this question

I am interested on buying a diamond but I am not sure if it's good.

Rectangular modified brilliant

7.28x6.37x5.16
2.01 cts

Depth: 81.0%
Table: 73%
Girdle: Thin to thick
Culet: None
VS1/F

What do you think, can you help me? Is this a good stone?

 

[valeria101]

That depth! RUN!

 

[Garry H (Cut Nut)]

It is not just my opinion that as you get up in larger pav / crown angles that you start to:
1. loose contrast
2. get more leakage

Not to that Dave Atlas's AGA charts line up too.

You can read more at www.cutstudy.com and http://www.folds.net/diamond/index.html

But if you are a diamond cutter wanting to make really nice looking stones - you are competing for rough with people who will by goods that make lousy 1.00-1.05ct and nice .95-.99ct.
What would you do - pay more or sail as close to the wind as possible?
I would want to be able to cut and sell good and bad stones -into different markets.

 

[Mara]

Hmm according to that chart, my 29.9 crown angle and 41.1 pav angle looks pretty darn good!

Right on the red/orange cusp.