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Crown Angle / Pavillion Angle

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poleary2000

Rough_Rock
Joined
Oct 28, 2002
Messages
30
Most sites do not publish the diamonds crown and/or pavillion angle. How can I calculate these?

Patrick
 

rsilvers

Shiny_Rock
Joined
Oct 26, 2002
Messages
251
You have to ask the seller for a Sarin report. Say you want to check the the stone with some cut advisor software and you would really appretiate it if they can fax or email the report. EGL certs have crown and pav height percent, which can be used in the HCA.
 

biscuit

Rough_Rock
Joined
Sep 2, 2002
Messages
67
Hi Poleary!

Great question...there's no way to calculate crown and pavillion angles...they need to be measured when the stone is certified, or after by a Sarin machine. A couple thoughts...

I didn't know that EGL included these measurements, but *almost all* gemmologists and appraisers that have posted suggest staying away from EGL. If you do a quick search of the meassage forum for things like "certificate" or "EGL", you'll see people like Rich, Dave, Garry, and Leonid all mentioning that AGS and GIA are the most stringent in their grading, and that EGL is considered to be a second tier lab. So chances are good that EGL might grade a rock higher than in should be.

So going on what the experts out here have said, I limited my search to GIA and AGS. AGS does have the pavillion and Crown angles right on the certificate; GIA does not. I looked at a few GIA rocks and had the Sarin analysis done on them...but I found that the Sarin measured the rocks quite inaccuratly, getting wild measurements for things that were already known on the GIA report. I asked the experts what was up, and they said *always* go with the numbers on the certification...they're more accurate.

Sooooo, since small variations in the crown/pav angles would change the outcome of HCA, I lost faith in the Sarin. My suggestion is to stick with AGS certed rocks. Yes, they might be a little more expensive, but at least you'll have the peace of mind, if you intend to purchase sight unseen, that the crown/pav angles are as accurate as possible, & that the HCA estimate is accurate. Plus, with AGS or GIA, you shouldn't have to worry that you're buying a rock whose grading has been "inflated". Just my .02 worth.

Happy shopping! Have fun! ~:praise:
 

rsilvers

Shiny_Rock
Joined
Oct 26, 2002
Messages
251
Biscuit. How do you know AGS does not use a Sarin machine also?

http://www.sarin.com/

Sarin lists them as a customer. I bet that do, although it might be a better model or better calibrated.

Here is my take on EGL (from what I heard, not from personal experience). I have been reading all I can but I only have about 15 hours of experience.

My local jewler uses EGL because, they say, they have fast turnaround whereas AGS takes 4-6 weeks. They also said they sometimes send a stone to several labs and use the lab that gave it the highest grade (yes, they told me this). This is why I have stumbled upon stones with more than one cert. I asked if they used EGL Israel and they almost barfed. They said they have seen stones from EGL Belgium that were two, even more, grades off. They use EGL NY and EGL LA and think they are fine.

Now I asked another local Jewler and he said he is a certified gemologist and has been doing this for 23 years, and he does not even trust EGL-USA -- and said I should use GIA or AGS if I care. This is a jewler that told me he would stay with a depth of 61 or less and a table of at most 59. Of course he only told me these things after speaking with me a while and hearing that I cared.

Now I am nearly certain that he would approve of an EGL rock if it looked good to him under his own inspection, but we guys cannot easily compare stones -- so going AGS or GIA seems like good advice. So did a rock go to EGL because it would not pass AGS standards, or did it go to EGL because the jeweler wanted to sell it this season and did not want to wait 4-6 weeks? Could be either.
 

LesleyH

Brilliant_Rock
Joined
Aug 28, 2000
Messages
506
Patrick:

The AGS (American Gem Society) DQD (diamond quality document) is the only 'certificate' that provides crown and pavilion angles and percentages. We WISH that the GIA would provide these too - even though we have a SARIN machine in house, it would sure make our lives easier, especially for those consumers who want to calculate the HCA.

LesleyH
www.whiteflash.com
 

rsilvers

Shiny_Rock
Joined
Oct 26, 2002
Messages
251
LesleyH,

How well does your Sarin machine correlate with the AGS numbers? And if they are not the same, do you assume that AGS is correct and your machine is wrong, or do you think either could be correct, but you have to sell based on the AGS document no matter what.
 

LesleyH

Brilliant_Rock
Joined
Aug 28, 2000
Messages
506
RSilvers:

SARIN is a photoimaging machine and is the #1 measuring instrument on the market today. The catch is that every machine has to be calibrated and there may be very slight differences between any 2 machines. In addition any presence of dust or lint on the diamond immediately affects the reading. This explains the minute differences that usually exist between any 2 readings of the same diamond.

We consider the AGS DQD to be the official document. However we do provide SARINS on all stones sold at Whiteflash.com.

LesleyH
www.whiteflash.com
 

Richard Sherwood

Ideal_Rock
Joined
Sep 25, 2002
Messages
4,924
Actually, there is a (trigonometric) way to figure crown angle & pavilion angle, if you know the crown height, table size, pavilion depth & culet size (if any).

Here's the formulas:

Pavilion angle-
degree corresponding to
If the stone has a culet, the formula changes to-
degree corresponding to

Crown height-
degree corresponding to

You can find a table of tangents at http://athena.ngdc.noaa.gov/globedev/tg/landcover/Tangents.html. You match the tangent up to it's corresponding degree of angle.

Pretty cool, eh?

Rich, GG
Sarasota Gemological Laboratory
 

kopper

Rough_Rock
Joined
Oct 3, 2002
Messages
7
Hi Richard,

This is interesting. I have a question for you. I'm assuming this formula would work for princess cut dimensions as well??

Thanks
 

scotch

Rough_Rock
Joined
Oct 15, 2002
Messages
94
Yes, you can calculate the angles - at least theoretically. Look up the formula in the thread titled "diamond dimensions", posted 10/19/02. This formula will give you the crown and pavillion heights. You can then calculate the angles using Rich's formulas. The bad news is: for the formula to work properly, you have to have precise girdle measurements (or at least a very close guess). Of course, if you have the precise girdle, e.g. from a Sarin report, you'll likely also have the angles. Anyway, if you should have the girdle, but not the angles, this way you can get a close estimate at least for the pavillion angle. Crown angles mostly come out less close, still haven't figured out why.

Scotch
 

rsilvers

Shiny_Rock
Joined
Oct 26, 2002
Messages
251
----------------
On 10/29/2002 7:56:11 PM

Actually, there is a (trigonometric) way to figure crown angle & pavilion angle, if you know the crown height, table size, pavilion depth & culet size (if any).



Not useful for people interested in HCA since that can already accept crown and pav height so if HCA is the goal, no need to compute the angles from the heights.
 

Richard Sherwood

Ideal_Rock
Joined
Sep 25, 2002
Messages
4,924
Kopper, the formula should work with princess cuts as well.

Scotch, the formula is independent of the girdle measurement. All you need to know is the crown height and table size, or the pavilion depth and culet size.

It's similar to measuring the height of a building if you know the distance from it to you, and the degree of angle from you to the top of the building.
The crown height is taken measuring from the bottom of the long, kite-shaped facet up, while the pavilion depth is measured from the top of the long pavilion main facet down.

These are the facets from which crown angle and pavilion angle are measured. They are in essence the same as the line-of-sight degree measurement from where you stand to the top of the "building".

Rich, GG
Sarasota Gemological Laboratory
 

poleary2000

Rough_Rock
Joined
Oct 28, 2002
Messages
30
So...based on all of this great information. Can I still calculate HCA if the only thing I have is a GIA?

Patrick
 

scotch

Rough_Rock
Joined
Oct 15, 2002
Messages
94
Rich,
maybe I should have been a little more precise: The formula that depends on girdle measurement is the one I posted earlier, and which you can use to calculate crown and pavillion heights. Once you have the heights, you can use your formula to calculate the angles, and this calculation certainly does not depend on the girdle.

Scotch
 

scotch

Rough_Rock
Joined
Oct 15, 2002
Messages
94
Well, you won't get a precise result, but you can sometimes figure out if the stone differs substantially from "ideal" proportions. For example, suppose you're looking at a 0.75ct rb stone, 6mm by 6mm, table 56%, depth 60%, medium girdle. The exact girdle will most likely be somewhere in the 1.0% to 1.6% range. If you assume a 1.0% girdle (measured at the valleys), the formulas will give you a pav angle of 42.8 and a crown angle of 27.1. A thicker girdle would concentrate more mass in the girdle, and thus you would get even steeper pav and shallower crown angles. This way, you could be pretty certain you are not looking at a well proportioned stone. However, if you are seriously considering a particular diamond, you should always get a Sarin report. This way, you won't have to do any guesswork.

Scotch
 

rsilvers

Shiny_Rock
Joined
Oct 26, 2002
Messages
251
Nope -- you cannot calculate the HCA from just a GIA. You need an AGS cert or a Sarin.
 

Richard Sherwood

Ideal_Rock
Joined
Sep 25, 2002
Messages
4,924
Ah, I see Scotch. Thanks for the clarification.

Poleary, the only way you could run the stone through the HCA is if you know the crown height and pavilion depth. For the life of me, I can't understand why one of the world's premier laboratories doesn't give this information. It makes them look like a dinosaur...

An interesting thing though is that you can find out that info if you call the GIA with the cert number. THEY TAKE IT DOWN BUT DON'T PUT IT ON THEIR REPORT!

Can you believe that?

Rich, GG
Sarasota Gemological Laboratory
 

Tarams

Shiny_Rock
Joined
Nov 26, 2001
Messages
228
Rich, GIA has crown height & pavillion depth??? Why wouldn't they put that on their certs??!! Looking like a Dinosaur, yes!
Does that mean I should call them for the stats on mine & not have a Sarin done?
 

Richard Sherwood

Ideal_Rock
Joined
Sep 25, 2002
Messages
4,924
The Sarin report is more in depth than just giving crown height and pavilion depth. It also gives the crown and pavilion angles for each main facet, as well as showing the thickness all the way around the girdle. It's a pretty nice report.

You can, however, get the crown angle and pavilion angle with the formula I posted, after getting the crown height and pavilion depth from GIA. It will be an average, but should work nicely.

I'm not a 100% certain that the GIA will give out the information to a private individual. I know they only issue reports to members of the trade. I would think as a "report owner" that they should oblige you. If not, email me the information and I'll see if I can get it for you.

Rich, GG
Sarasota Gemological Laboratory
[email protected]
 

biscuit

Rough_Rock
Joined
Sep 2, 2002
Messages
67
rsilvers -

great point about AGS...yes, they do use the Sarin to grade their rocks. But for whatever reason, across the board, cutters and gemmologists will defer to the measurements on the AGS cert over a local Sarin. I don't know if AGS cleans their machine more often...they use a machine that's available to anyone who wants it, so I don't know why people in the industry regard AGS's reading so highly. Like I said, the only reason I got nervous with Sarin is because it was way off on things that were certain, such as diameter and such (hand measurements almost always are more acurate), as well as depth and table %.

I have heard from a few people that the EGL labs in the US are more consistent...but like you said, it's hard to know. I don't know why any given rock finds it's way to a particular lab...and experts out there know how the trade works???

thanks all! Good discussion! ~:praise:
 

schrum

Rough_Rock
Joined
Apr 6, 2003
Messages
19
Hello,

Could someone repost the formula for Crown Angles - there seems to be something missing from them and I cannot figure out how to use them...

Thanks.
Michael

Ps- maybe I am just stupid - please tell me if I am !
 

Richard Sherwood

Ideal_Rock
Joined
Sep 25, 2002
Messages
4,924
Mike, following is the trigonometric formula for determing crown and pavilion angles, along with an attachment containing the chart of tangents used in combination with the formulas.

In order to determine the (average) crown angle, you need the crown height and table size. (Note- GIA does not include crown heights on their reports.) In order to determine the (average) pavilion angle, you need the pavilion depth and culet size, if any. (Note- GIA...)

CROWN ANGLE:
Tangent (crown angle) = 2 x (crown height%) / 1 - table%

PAVILION ANGLE:
Tangent (pavilion angle) = 2 x (pavilion depth%) / 1 - culet%

For example, let's say a diamond has a 14.8% crown height and 57% table:
2 x 14.8 / 1 - 57
29.6 / 43
0.688

0.688 falls 60% of the way between the tangent of .67 & .70 (see chart).
The .67 tangent corresponds to 34 degrees, and the .70 tangent corresponds to 35 degrees. Hence you would add .6 degrees to 34, coming up with an average crown angle of 34.6 degrees.

This estimation corresponds closely with DiamCalc's estimation of 34.5 degrees. Usually when you use this formula you will come within .1 degree plus-or-minus.

Tangents.JPG
 
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