How Inaccurate are Crown/Pavilion Percentages?

[finchuck]

If a stone scores 1.7 on the HCA using crown and pavilion percentages, could you confidently say that it''ll be a performer? How much of a difference could the percentages make compared to the angles?

Also, are the depth, table, crown and pavilion figures given on an EGL-USA cert usually accurate?

 

[Mara]

If the stone scores a 1.7 on the HCA with %s...I would venture to say that with angles it would be more like 2.3-2.5. I have seen swings like that more often than not when it comes to % vs angles. Angles are more accurate, %'s are using a range...so you may get better results than with the spot-on angle.




It should be a good performer if its in the 2 range...but it won't be under 2.0 with angles most likely. 99% sure.

 

[finchuck]

Thanks Mara... from what you said, then what the hell is the point of percentages?

 

[Mara]

It gives you an *idea* of what the stone will perform like on the HCA with angles...and the EGL certs were not created with the HCA in mind! Luckily the HCA does take %'s so you at least get a feel for it...for example if a stone is a 5.2 with %'s...then chances are that with angles, it would be even worse, such as a 6. So you would know to completely discount that stone. Not bother with it any longer. So it DOES help to have %'s if you can't get angles, but for the FINAL decision IMO I would get the angles..and/or only stick with stones that are in the low HCA scores (e.g. 1 on %'s would be a good bet on angles IMO) if you can't get more information.

 

[oldminer]

When you have exact diameter measurements and a percentage measurement of the table, crown and pavilion, you readily and accurately may calculate the crown or pavilion angles with a trig calculator and a little engineering education.

I don't have the formula at hand, but I have done this many times with a calculator and it is not rocket science. The HCA was made to use angles and I found it easier to use percentages when I made the AGA Cut Class charts. If I used the "correct" angles instead of percentages, nothing would occur with the cut class. While the HCA is sensitive to this, I am not at all certain why that happens. Garry Holloway may come by and explain it to you.... In a perfect world these numbers should be interchangeable. I imagine it is the way the machines calculate the percentage or measure the angle. One method is "looser" than the other.

 

[valeria101]

Trigonometry aside, I found HCA scores based on angles and percentages taken from Sarin and/or certs of the same stone to vary by up to 1 unit in EITHER direction. So, the measurement based on percentages is only less precise, not consistently biased.

The use of angles is preferred from two reasons though: the HCA was built to use it, and some diamonds were found to be cut such that they would score deceptively better by percentages rather than angles. As far as I remember, these stones should have some telling signs, such as fair symmetry, large culets or very thick girdles.

No matter whether you are using angles or percentages, the HCA does not account for cut details critical for light return (such as the cut of minor facets). Using HCA is good for a first cut, but is not (nor it was intended to be) the ultimate in diamond optics testing. This is there are other tests available to communicate cut quality online (the Ideal Scope, and others...).

Hope this helps...

 

[aljdewey]

The reason for such variances in percentages? They are typically rounded to the next whole number, and that can make a huge difference.




Under a rounding system, $1.51 and $2.00 would both be consider $2......but on amount is a full 25% less than the other. That's a BIG swing.

 

[pqcollectibles]

About a year ago, a couple of different formulas were posted to calculate angles from %'s. Using the formulas to calculate angles from %'s on diamonds with known Sarin info, the forumlas didn't work.

I hope Mr. Atlas will post the equation he uses. It would be a great help for everyone!

In my particular case, my earrings each score 0.9 using %'s and 1.2 using angles. My ring scores 0.7 using %'s and 1.3 using angles. Yesterday, Lara posted specs for her diamond. Her score comes out the same using either angles or %'s. That was a first for me!

 

[finchuck]

What do you think about this diamond? Clich HERE for a link to the EGL cert.

BTW, I found this formula to convert percentages to angles on this forum, I'm not sure if it's correct though. It's HERE. I think, if I did the calculations correct, that using the angles from that formula the stone above scored a 2.3 on the HCA.

Also, I noticed there is an AGS Appraisal at the bottom. Does every EGL cert include this?

 

[angela]

Have you ever seen this? My diamond comes out as a 2.1 using angles, and 2.2 using the %. Very strange. Even though it is just above a 2, it is still very sparkly and a good performer in every light I have tested (so far).

 

[pqcollectibles]

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On 2/27/2004 3:58:07 AM finchuck wrote:

What do you think about this diamond? Clich HERE for a link to the EGL cert.

BTW, I found this formula to convert percentages to angles on this forum, I'm not sure if it's correct though. It's HERE. I think, if I did the calculations correct, that using the angles from that formula the stone above scored a 2.3 on the HCA.

Also, I noticed there is an AGS Appraisal at the bottom. Does every EGL cert include this?----------------

That formula was tried and found to be not totally accurate.

For example, here's the specs on my diamond.

Table 55%
Total Depth 60.5%
Crown 15.5%, 34.8 degrees (sarin), 34.5 degrees (calculated)
Pavillion 42.9%, 40.8 degrees (sarin), 40.6 degrees (calculated)

%HCA 0.7
True HCA 1.3
Calcuated HCA 0.6

 

[fire&ice]

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On 2/27/2004 9:10:45 AM angela wrote:

Have you ever seen this? My diamond comes out as a 2.1 using angles, and 2.2 using the %. Very strange. Even though it is just above a 2, it is still very sparkly and a good performer in every light I have tested (so far).----------------

Of course it's sparkly. For Pete sake - the under 2 mark is *completely* arbitrary & does not account for any margin of error nor other important factors. I have stones that score .5, 1.2, 1.7, 2-3 & 4+. Which one do I prefer? - the biggest one, then the next biggest one, etc.

Also, percentages are not always rounded off. Mine were not on my cert. Yet, I do see it on some certs. I can't explain why.

 

[angela]

Just goes to show that you shouldn't rely solely on the #'s, take a look because stones that score lower can still be spectacular!!

 

[finchuck]

What do you think about this rock, it's EGL certed:

Weight 1.67 ct.
Shape Round Brilliant Cut
Color I
Clarity SI2
Measurements 7.7 - 7.65 x 4.62 mm
Depth 60.2 %
Table 60.0 %
Crown 13.8 %
Pavilion 43.0 %
Girdle THIN TO MEDIUM FACETED
Polish GOOD
Symmetry GOOD
Fluorescence NONE
Culet NONE
Appraisal Value $12880.00

 

[Mara]

A stone that is 2.1 or 2.2 or even 2.8 on the HCA is still going to be beautiful 99% of the time. My stone is 2.8 on the HCA and it's very sparkly and beautiful. It was 1.5 when we bought it, and would NOT have bought it if we knew it would score 2.8...but oh well.

It doesn't mean it's less sparkly at 2.8 than at 1.5.




It's more when you get to the 3+ range and sometimes more around 4+ range that things get sketchy. But many times people are looking for under 2.0 because they want to make sure everything matches up in their minds and they are buying sight-unseen.

 

[finchuck]

I just got a pic of the stone I mentioned above, the I/SI2. I believe this is 10X loupe, and I can see a tint of yellow face-up, do you think this is visable to the unaided eye? It also appears "cloudy" in the center. I think it's a dud.

 

[pqcollectibles]

Fin,.... You really need to get the Sarin info for this diamond.

http://diamonds.pricescope.com/60.asp

In 60:60's only a small range of angles combos yield good performers.

 

[aljdewey]

Fin.....is this another ebay diamond?




What happened to the promise ring plan?

 

[Giangi]

I hope it's just a bad picture, otherwise it looks like a M-N color stone. Did you notice the big crystal in the middle of the table?

 

[pqcollectibles]

I was just thinking the table looked like the Home Plate of a baseball diamond!

 

[Mara]

Fin you are confusing us

First it was a low budget for a decent sized stone. Then it was a promise ring while you waited. Now a 1.67c stone for $12k?! Wow! Did you come into some $$?

 

[fire&ice]

----------------
On 2/27/2004 4:44:29 PM Mara wrote:


Fin you are confusing us

First it was a low budget for a decent sized stone. Then it was a promise ring while you waited. Now a 1.67c stone for $12k?! Wow! Did you come into some $$?
----------------

This made me chuckle. I'm a little confused too. But, the 12k is more than likely the "appraisal" that EGL certs come with. Without a doubt that figure is at least double what it's trading for.

I really can't tell from that picture. But, if the color was accurate - that's one yellow hued stone. My I is not even in the same yellow ballpark.

Fin, I think we need an intervention w/ you and ebay.

 

[aljdewey]

----------------
On 2/27/2004 5:07:56 PM fire&ice wrote:

Fin, I think we need an intervention w/ you and ebay.

----------------

Ooooooooooo........can I volunteeer??!!!

 

[finchuck]

LOL, I know I know, I bounce from here to there when it comes to these purchases. As far as the promise ring is concerned, I ditched that idea and bought her a Coach watch for Valentine's Day. She absolutely loved it.

Anyway, I'm constantly looking at diamonds on ebay and a couple of them "tweak" me from time to time. I'm not planning on proposing in the near future so in the meantime I'm always cruising from site to site. The figures on this one were nice, but when I saw the 10X photo I was a bit turned off. The seller claims that it's completely eye-clean and faces up white to the unaided eye and that the appearance is only due to the magnification. That seems unlikely to me, is it possible?

 

[fire&ice]

----------------
On 2/28/2004 3:18:05 AM finchuck wrote:

LOL
The figures on this one were nice, but when I saw the 10X photo I was a bit turned off. The seller claims that it's completely eye-clean and faces up white to the unaided eye and that the appearance is only due to the magnification. That seems unlikely to me, is it possible?----------------

It may look eye clean; but, I suspect this diamond has transparacy issues because of the large inclusion smack in the table.

 

[finchuck]

The auction is lingering around 2 grand. Do you think it'll look like crap to the unaided eye or is it worth a thought?

 

[Garry H (Cut Nut)]

I have been travelling and apologise for not being uo to date with all the threads.

The reasons why %'s do not wark are as stated they are usually rounded.

But the main reason why - even if you have numbers to .1% are that the pavilion is the most critical for cut quality, and because most stones have a tiny culet, the depth % measurement needs to have a little added to it to do the trig calculation. So the angle measurement usually is understated.

In addition we prefer results from Sarin because Sarin actually measures the angle (the dimensions and %'s are calc from the angles of each facet plane) where as Ogi got around Sarin's patent by using the less accurate method of calculating angles. Ogi is however better at measuring stones, but that is usually not such a big issue.
But this is also 1 reason why Sarin is not as efective at measuring table size. However a 1-2% table variation has about the same effect as a 0.1% pavilion angle effect.

So does that answer the ?'s

 

[finchuck]

Definitely. I'm now understanding why it's so important to have the angles.