HCA Assumptions

[magpie2]

Hi
This is my first post, and before I cautiously begin my questions, I''d like to say thanks to all the regular contributors to the site for the enormous amounts of helpful advice offered here. I came across PS recently while part way through my first diamond purchase and found the information invaluable in choosing a stone. Fact is I''ve become intrigued by some of the finer details of "ideal cuts" and so on, and as I''m in the process of making a second purchase now, which I''d like to get as close to the bullseye as possible, I thought I''d ask for views on one or two issues to help me do that.
First the HCA (- thanks Garry!). Am I right in thinking the system takes into account Jasper Paulsen''s work on girdle thickness? If so this does this explain why the first stone I bought had an excellent HCA score (0. despite falling a little short (CH14 CA33.1 T57 PA40.75% D60.5) of the seemingly "preferred" 34/35 CA numbers and ACA/GIA/AGS top proportions. The other thing I''d like to know is just what sort of difference (to the eye) it would make choosing say a 34.5CA (would that be about a 15%CH?) if the other numbers stayed the same?
Finally, can anyone tell me does it make a significant difference to the key numbers if a diamond is in the larger range - say 10mm plus?
Thanks.

 

[lisa1.01fvs1]

Use HCA if its a virtual stone.

Use your eyes and #''s too if its viewable IRL.

 

[Stephan]

If you choose a diamond with a higher crown, the diamond could have better light return.
But don''t worry, I also love shallower diamonds. I often think they have a better scintillation, and the fire is more visible to my eyes (I''m one-eye blind, no stereo vision).
You have to compare diamonds to make out what is YOUR taste.
If you chose a well cut 40.8/34.5 diamond, it will probably please everyone, but you might prefer deeper combo''s (41.2/35) or shallower combo''s (40.5/34). That''s a matter of taste. Everybody is looking for something when talking about diamond beauty, and that can be very different.
Lower halves length and star length are important too, but the HCA doesn''t use them to calculate the score.

 

[magpie2]

Stephan
Thanks for the reply. We''ll try to view diamonds "live" in the UK but will probably end up buying on line. Not likely to go for a steep/deep but a little concerned we may not be able to find the required combo''s in the shops. Sounds like you''ve seen a few stones, so would you say you can really see (with unaided eye) more scintillation / fire in a 34/40.5 for example vs a 34.5/40.75, given equal cut quality. I sort of understand the scintillation bit, but I got the impression fire was more associated with higher CA''s.
Furthermore, I know you shouldn''t "buy the paper" but eg. AGA "top" crown angles start at 34.3 and I''m sure there''s a reason for that.

 

[Stephan]

Hi Magpie,
You are welcome, but that''s only my opinion.
The shallowest of my diamonds has a 33.7° crown, 40.7° pavilion and 58% table.
When talking about fire, this one has tons of fire under spot lighting, especially a lot of strong light blue, also a lot of red. Fire I saw in steeper diamonds was not that captivating, much more orange, yellow, ...
I''m buying a quite similar diamond right now (LINK), even shallower as you can see.
I don''t care if it''s only a "very good" cut for GIA, because I''m sure it will look better than 99% of the 34.5°/41.8° GIA "excellent" cuts outside there; it will also have a better price and a better spread.
About minor facets, you can find some interesting videos on GOG website.
Good luck in your diamond hunt!