pavilion and crown angle -- is this person full of it?

[dstroock]

I spoke with an appraiser today who said (when I asked about getting a Sarin report) that they could figure out my crown and pavilion angles via a "mathmatical formula" and if I brought my stone in, they''d do that for me. Somehow that sounded a bit fishy, since if it were true -- wouldn''t it be posted someplace? the diamond I''m looking at is 1.58 carat 61.2% depth, 60% table, and 7.43-7.57X4.59mm (thin to slightly thick girdle and no culet. I''m a bit worried about the table size. Any thoughts? Its going to take a bit of work and money to get to see the diamond in person, and I''d like to avoid doing that if the cut just isn''t that great.

 

[squirerad]

A mathematical formula to calculate the crown and pavilion angle from other known factors does not seem out of the realm of possibility. So the answer to your question is I don't know. But I'm posting to give my opinion that you should get a true Sarin report.

Plugging in numbers can give you an approximation, but how would you know that it is exactly the real thing? Sarins give you an exact angle (plus or minus a small error) for not just one of the crown or one of the pavilion angles but all 16 of them so that you see how precise, i.e. how much variance there is, from say a pavilion angle of 40.8'.

Even without the crown or pavilion angle, we already know that a table of 60% is already not of the range for an ideal cut.

squire

 

[Mara]

do a search on this subject...it was discussed in the past. from what i recall it may be possible but it's pretty technical and would just be easier to find a sarin machine!

also i don't know how on the mark they'd be to try to 'calculate' it.

 

[Magnum]

It is a mathematical impossibility to calculate crown and pavilion angles from the information you provided (table, depth, girdle, culet size, mm measurements). If you had that information you could estimate the crown angle/percentage if knew the pavilion angle/percentage or vice versa with some basic trigonometry. It wouldn't be exact, but it would be close. You can't figure out both, though. Too many variables. With a given depth and table there are an infinite number of pavilion and crown angle combinations. It's the common mathematical problem of not having enough equations for the number of variables. Now, there are old ways to estimate crown and pavilion angles that people used to use before there were sarin machines, which I'm not too knowledgable about, so that may be what he's talking about. it's not super accurate, but may be good enough. Not too sure about that. i can pretty much guarantee that he won't be able to figure out the angles using just math and the numbers you provided.

 

[moremoremore]

I think the real problem is not IF they can do it....but the fact that they said they could do it IF you bought the stone.....Well then, if they can, why don't they before you buy it. Ridiculous. Find another vendor.

 

[hoorray]

I've always heard you can't calculate the crown and pav angles, but don't know that for sure.




About the table -- it's a bit large. Falls outside of the ideal range, but doesn't mean the stone is a dog. (There was another post about a 60% table this am that Valeria posted an CHA chart for so look around.)




However, if you are jumping through hoops to view a suspect stone, why don't you regroup and take a different approach. The first big stone we bought recently we had sent to an appraiser where we could see it. Of course, with nothing to compare it to, and our inexperienced eyes, it was beautiful, and we bought it. In hindsight we lucked out, since we really didn't know what we were looking at/for, but had done our homework on specs and gotten opinions here, so really it is beautiful.




Recently I sent stones to Rich Sherwood to "be my eyes". He certainly knows more than I do, and it worked out great. I didn't have to drive 200 miles twice to visit the appraiser, and Rich gave me a great detailed report and some purchase advice.




All that said, if I were in your shoes, I would try to find a stone that had more info on it so that I could do the initial screen with complete info -- pre-purchase. Then send it to someone who will give you a good work up and sound advice.

 

[valeria101]

Yeah... right!

There is some relation between table and depth, but it is not precise enough to go straight to the HCA (or any light return-predicting device, 'cause this is the least data demanding of them) with it. If you can live with about 2 degrees approximation for pavilion and crown, why not! Only that kind of range gets you right out of the HCA chart...definitely not precise enough for predicting briliance and if not, what's the point?

Basically, it's not worth it. If it comes to savings, the percentages on a EGL report are much better approximation.

However, I'd ask for the formula

 

[Magnum]

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On 5/4/2004 9:26:02 PM lop wrote:

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I second that.

lop gives some good advice.

As far as the table size, i wouldn't be too worried. Some people actually prefer a little bit larger table. A stone with a 60% table can still have good crown/pavillion angles. It's just impossible to know without a sarin. A couple things that do raise eyebrows a little are the girdle thickness and the diameter measurements. Thin to slightly thick, while not terrible, suggests a little bit of a wavy girdle. As far as the diameter measurements, there is a .14mm difference between the min/max numbers, which suggests that the diamond is slightly out-of-round. While neither of these facts means the diamond is definitely going to be a dud, it does hint that the craftsmanship used to cut it was not the absolute best. If you're looking for the super-ideal cut, H&A type stone, this is probably not it. Admittedly, Pricescoper's tend to be a little picky, myself included. You tend to get sucked into analyzing every little thing. While this stone may not be a super-ideal cut, H&A, best of the best stone, it may still be a beautiful stone. As long as your comfortable with that, you may be able to get a good deal on a good diamond.

 

[Garry H (Cut Nut)]

You need a sarin to get the angles - full stop.
The stone owner at time of submission to GIA can request the angles from GIA.
Or for $25 you can buy an ideal-scope and check for your self to get your final selection - ask the vendor if he has one.
But if you have doubts about the vendor then please use an appriaser.

 

[valeria101]

Oh well, I was looking for IGI and EGL stones... since the resepective certs hold the right cut numbers and a reasonable discount

However, after EGL cert #5 I got stuck! Four out of five stones fall ridiculeously close to the HCA's seal of approval, but not quite. Actually, they fall so close, that the full part of the glass starts to apear tempting: what if these are ok-ish after all ?

The common denominator of the four stones is a rather lower pavilion (12%-13%) and 'excellent' spread regardless. On the HCA chart they would fall somewhere inside the black contour... obviously no one wanted them to get an unapealing AGS cut grade, and why EGL is another story.


What gives?

 

[valeria101]

 

[Richard Sherwood]

-----------
I spoke with an appraiser today who said (when I asked about getting a
Sarin report) that they could figure out my crown and pavilion angles
via a "mathmatical formula" and if I brought my stone in, they'd do
that for me.
-----------

This is possible using tangents and trigonometry if you know the (average)crown height and (average) pavilion depth of a stone, along with the table size and culet size.

It actually is pretty accurate if the measurements you're using are precise. I compared it several times to the DiamCalc estimates, and they were usually close or dead-on.

A Sarin or Megascope report would probably be an easier and more informative way to go, though.

 

[Magnum]

Rich,
I agree. My point was that with just GIA report info, i.e. without the crown angle or percentage and without the pavilion angle or percentage, it's impossible to figure out any of the four numbers. As you say, to go from angle to percentage, and vice versa, is relatively easy and should be somewhat accurate using trigonometry and a scientific calculator. Even if you just had say, pavilion depth percentage, you could figure out a rough estimate for crown percentage by subracting girdle thickness from total depth percentage (assuming no culet), and then with pavilion and crown percentages, you could roughly figure out pavilion and crown angles. But you need at least one of these numbers (pavilion angle, pavilion depth, crown angle or crown height) to get started figuring out the other three. If you don't have any of the four values, it's mathematically impossible to figure any of them out with just table and total depth percentages.

 

[Richard Sherwood]

Yep, true enough Magnum. It couldn't be done with only the figures supplied on a GIA cert, for example.

But the appraiser could supply the missing figures in order to use the equations. He'd just have to measure the crown height and pavilion depth.

I mention it only so dstroock wouldn't think the guy was "full of it".

 

[Garry H (Cut Nut)]

Dstroock you could help the jeweler out by enlightening him (or her) about the ideal-scope - then angles and %'s are irrelevant

Many out there still do not know