Q: Crown/Pavillion angle variance and the HCA

[antigoon]

It seems that most of the diamonds I've seen lately have had their crown and pavilion angles right along the "ideal cut cliff", where the diamond's HCA score can fall from 1 to 3 with the slightest variation in crown and pavilion angles.

/idealbb/files/t563.gif

[/p]Table : 56
Crown Angle : 35.1 (35.0-35.3)
Pavillion Angle: 40.9 (40.8-41)

[/p]

If I plug in the average angle values (35.1, 40.9), I get a respectable HCA score of 2. If I plug in the minimum values (35.0, 40., I get 1.4. But if I plug in the maxima (35.3, 41), I get an HCA score of 3.

(Again, I understand that the HCA is a rejection tool, not a selection tool. My question #1 below is about whether I should reject diamonds that have a “worst-case” HCA >2.)

/idealbb/files/t56.gif

I’ve got three questions:

1. Am I justified to be worried about any diamond whose proportions “fall off the cliff” like this?
   
2. There is a margin of error in the Sarin and other measuring machines. Unfortunately, this margin is not listed on the reports I’ve seen to date. (This is too bad, because the more expensive Sarin machines are more accurate than the less expensive ones, but if I don’t know what kind of machine it is, I have to assume it’s the less accurate one.) So should I take an additional ±0.1 degree to account for possible measurement error when measuring HCA, “just to be safe”?
   
3. Why are there so many diamonds that are right on the edge of the cliff? Is there something inherent in the geometry that makes it easier to cut a diamond that way?

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[aljdewey]

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On 6/26/2004 11:33:15 AM antigoon wrote:

It seems that most of the diamonds I've seen lately have had their crown and pavilion angles right along the 'ideal cut cliff', where the diamond's HCA score can fall from 1 to 3 with the slightest variation in crown and pavilion angles.

[/p]

/www.pricescope.com/idealbb/images/smilies/3.gif[/img]/www.pricescope.com/idealbb/images/smilies/3.gif[/img]/www.pricescope.com/idealbb/images/smilies/3.gif[/img]why are there so many diamonds that are right on the edge of the cliff? is there something inherent in the geometry that makes it easier to cut a diamond that way?

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So many diamonds push this envelope because cutters are striving for weight retention while still meeting the ideal-cut criteria.

Obviously, a diamond with a 41 pavilion angle has more depth (and therefore more weight) than one with 40.7 angle. Similarly, a diamond with a 35.3 crown has more height (and therefore weight) than one with a 34.6 crown.

Do you need to worry about those that fall off the cliff? It depends on what you want. If you really have to have a diamond with tight specs, then stay with 55-56 table, 34.3-34.7 crown angle, and 40.6-40.9 pavilion angle. Then, even allowing for possible machine variances, you're still likely to be in the sweet zone.

If you are looking for a diamond that's on fringe (perhaps better value), then I would consider those "just off the cliff" and examine them carefully.....make sure they don't have leakage ring around the table, etc.

Good luck.

 

[Dancing Fire]

i agree with aljdewey.just to be safe,i also prefer those crown angle and pavil angle combo. if i were to buy a stone today, i wanna make sure it's still an ideal cut when GIA comes out with the cut grade system. those stones with a crown angle of 35.5-35.8 combined with a pavil angle of 41.0-41.2 may not be an ideal cut next year according to one of the cutters that post here on PS.

 

[Paul-Antwerp]

You are right in your observation, Antigoon, and I have to share your fears.

Since about a year, I see some very good brands now having on average stones with a pavillion angle of 40.9, where that used to be 40.6. At the same time, their crown angle has also gone up.

I think that this has happened because of two causes:

1. The HCA is widely used on these stones, and as long as the averages are still within the good area, the HCA-score is still great. This is another reason, why the HCA should be used as a sorting-tool and not as a decision-tool. The HCA-score gives an idea of the theoretically best possible performance, not of the actual performance.

2. We are leaving an era of over-supply of diamonds, and now we are in a period of not enough rough diamonds being available. Cutters are under enormous price-pressure, and cutting in this way saves some weight, and thus saves some profit. For those brands, who do not cut themselves, they have more difficulty selecting only the best possible cut diamonds. Their supplier will probably force them to also take some stones, that they would not buy a year ago.

Personally, I am happy to have organized a very lean organisation, and to keep all cutting under close personal control. Because of this, we can keep up our consistency, and most importantly, we do not have to compromise on any detail.

On the other hand, I fear that GIA, under the influence of big cutting houses and based upon flawed research, might reward such stones, which are cut slightly deeper, with their highest cut-score. Their first presentations have been very unclear, but they do point that way.

Live long,