Cut Adviser & Pavilion Angles

[stook1]

Hello,
Thanks everyone for the fantastic contributions on this site. Without this resource I would be very lost in the diamond buying experience. Actually, I probably am still lost and know enough to be dangerous (at best).

Now for my question... In searching and reading I have read in one thread or another that one should avoid diamonds with pavilion angles greater than 41.5 and that even diamonds with pavilion angles as low as 41 may not be ideal due to averaging and lack of proportion which can result in light leakage for the facets with a steeper angle than the average. My sincere apologies if this is either wrong or a gross over simplification.

What I am wondering is if I am shopping 2 diamonds on the internet both with a Cut Advisor rating below 2.0. Without respect for the Crown angle, should I focus first on the stone that has a pavilion angle of say, 40.7 vs. the one that is at 41.2 or even 41? Or should the pavilion angle only be evaluated in conjunction with the crown angle and table size?

thank you!

 

[Regular Guy]

Hi Stook,

First off, I may be a bit orthodox vs. typical...so I''d go with satisfaction with the combo.

Because of that, based on shopping the internet, I''d tend to let the search by cut db do the math for you, and just select from there.

That being said...rather than worry how to fashion this data, you''ll note that if it''s not already AGS0...or GIA Excellent for that matter...populating the HCA chart with both crown & pavilion data tells you pretty well how both systems apply their evaluation...beyond Garry''s 0 - 2 run.

Otherwise...yes...pavilion angles do influence the most. Unless you''re looking for a short cut of some sort, that''s what I say...

 

[stook1]

Thanks for the reply Ira. I have used the Search by Cut function but unfortunately that has a dramatic impact on selection and there are obviously many other options from which to choose.

So I should not be worried about rounding, etc. (particularly with a GIA cert) with a pavilion angle at or above say 41.2? Should this be an irrelevent consideration in the screening process given the choice of calling in one of two very similar stones for further testing (ie. one with a 41.2 vs. one with a 40.7)?

 

[pricescope]

>>Or should the pavilion angle only be evaluated in conjunction with the crown angle and table size?

That is correct.

When HCA was developed back in 2000, it used system of penalties. In particular, light leakage visible with firescope/ideal-scope caused penalties in HCA. See e.g. http://www.diamond-cut.com.au/06_firescope.htm. Also, inverse 4:1 relationship between crown and pavilion angles has been accounted.

Therefore, if you take Classic Tolkowski proportions 34.5 crown and 40.75 pavilion angles as a reference, you need to adjust crown 4x in opposite direction to compensate deviation of pavilion angle. In your case, to compensate 41.2 deg pavilion you've got to have crown angle decreased 4x0.45=1.8 degree i.e. theoretical crown angle should be about 32.7 deg.

But this is a theory. You don't have to be so strict in the real world and have some room for deviations. Besides, small light leakage can be negligible if you have open setting when light can enter pavilion. (see e.g. this small survey)

 

[stook1]

Pricescope, thank you very much. That perfectly answered my question. I also found the video most revealing!

thanks!