comparing a super-ideal diamond using HCA

[jechow]

I am looking to buy a super-ideal diamond from an online vendor with the following traits:

Carat: 0.803
Color: F
Clarity: SI2
Depth: 60.9
Table: 56
Crown Angle: 35
Crown %: 15.4
Pavilion Angle: 40.9
Pavilion %: 43
Girdle: Faceted 0.7% to 1.5%
Polish: Ideal
Symmetry: Ideal
Culet: Pointed

My concern is that the HCA (Holloway Cut Adviser) evaluation determined all four measures/factors at very good, NOT excellent.

Should this be a concern or is this evaluation tool "inaccurate" when evaluating super-ideal diamonds?

Thanks

 

[highendgems]

.7% girdle is a bit too thin for my liking. stay away from this stone-it is not super ideal.

 

[Garry H (Cut Nut)]

You can drop to 1/2 that girdle thickness before getting Very Thin, High End.

The stone is getting into slightly steep and slightly deep proportions - but depending on other factors will still be a very nice stone.

 

[aljdewey]

Hi, JE:

A few things. First and foremost, take the comments from Highend with a HUGE grain of salt. He has been populating this board of late with a bunch of erroneous information.

Second: The diamond you asked about is this one from WF, and here's how it scores on the HCA:

0.803, F, SI2, 0 (H&A)
A Cut Above, 1.8-EX
ex-ex-vg-vg, 60.9, 56, 35°, 40.9°, AGS, f 0.7%-1.5%, pt, id, id, neg, $3192*‡

It gets two ex and two VG. (This may differ from what you got manually, and Garry explained that to me...the HCA takes girdle info into account. Pulled as part of a PS search, it has that information, but when you put the numbers in by hand, for the sake of simplicity, it doesn't ask you for that info.)

Second....it's also important to note that most diamonds only score VG on the spread until it's a shallow cut diamond...so it's a trade-off. You can get a bit more diameter at the sacrifice of fire. Most superideals score VG on the spread.

This is a good price for a stone that should perform BEAUTIFULLY. I think you've found a winner.

There are two things you can do to further satisfy your mind. One, talk to WF...ask Brian to tell you about the stone while he looks at it. Two, get it independently appraised--they'll even let you send it to an appraiser for approval before purchase.

Good luck, and let us know if we can help.

 

[jechow]

Hi aljdewey

What do you think about Cut Nuts comment "The stone is getting into slightly steep and slightly deep proportions..."? Could it be possible that this diamond was cut slightly deeper to achieve the 0.8 cart status vs 0.79 cart status?

Thanks

 

[aljdewey]

JE - Anything's possible....here's what I think.




The stone scores a 1.8 on the HCA. That means it has characteristics that outperform 95% of other diamonds on the market. It is not overly deep at 60.9. What Garry is saying is that it is at the upper end of ideal tolerances. The pavilion angle, depth, and table here would perform best with a 34 or 34.1 crown angle, and this crown angle is steeper.....35.




Do you know anything about photography? If you do, then you'll know that there are many combinations of apertures/shutter speeds that all produce the same "exposure". It's a compensation system...for every stop down in aperture, you have to go up a stop in shutter speed. Kind of like the scientific "for every action, there has to be an opposite and equal reaction".




The same is true of facet arrangement. For every increase in crown angle, there needs to be a proportionate decrease in pavilion angle to continue producing "optimal light return". What Garry refers to......when diamond exhibit steep crown angles and are fairly deep, there is the potential for a ring of light leakage under the table.




However, that doesn't appear to be a serious concern here given the rating of the stone. Of course, the numbers are only a way to weed out stones. The best way to get a better feel is to get an ideal scope image....that will SHOW you where there is leakage, if any. An independent appraiser's OK would be a great stamp of approval. I certainly would consider this stone worthy of your time to check into.

 

[Paul-Antwerp]

Hi,

Because the combination of a slightly too steep crown and a slightly too deep pavilion, I would call this stone an 'ideal' (well within AGS-ideal-proportions), but not a 'super-ideal' (the best combo of proportions).

That does not mean that the stone is bad, on the contrary, it is still a great performer and much better than 95% of the stones available on the market.

Also, it is very difficult to produce a 0.80 with super-ideal proportions. Technically, it is no problem. Economically, it is, because the same rough stone would probably yield a 0.90 with good to average proportions, which is more valuable than the 0.80 super-ideal.

Furthermore, the price is quite good, so no need to worry there.

Good luck on your purchase,

Paul

 

[Stephan]

Nice said, Paul.
Glad to see you again!