Question about Pav angles - Is it better to find a stone with all 8...

[Dancing Fire]

pav angles of less than 41 degrees or just the average of less than 41 degrees? Some of these range between the max and the min is pretty wide. Same with the crown angle.

 

[Magnum]

There's been debate over this before, between some of the experts. If you do a search on variance, you might find something. My personal preference would be to find a stone where all the angles are pretty close to the average, i.e. a stone with tighter variances. Less guessing involved. Some experts argure that just because there is a larger variance it doesn't necessarily mean it will be a bad stone. They argue that it could be because of a slightly tilted table, since sarin machines measure the angles with reference to the table instead of the girdle, and therefore the variance isn't as big of a deal in this case. They say that other tests like idealscope pictures are more important. Other times the angles are just all over the place. I think I've heard Niceice make the argument of why pay for an AGS0 stone that has angles that fall outside the AGS0 box, but the averages are still AGS0. Males sense to me. I don't think it's imperative that all the angles are below 41 degrees, as long as it's just a couple and it's not too far above 41. Maybe some of the experts can chime in.

 

[valeria101]

Leonid just made this kind of guess easy with those colorful charts next to the HCA

(example borowed without permission below). You can go on with the same kind of approximation exercise for the stones you consider (since then you'd know the table measure and use the sarin results to point the mid of the 'range of error'.

According to the HCA chart, you may want the angles to (predictably)vary just so that the measurements of the stone fall 'on the red' - actually in one of the two areas (magenta rectangles, labeled 1 and 2). So you can have variation of as much as one degree (and the Sarin of those H&A stones shows variations of less that that!) and still stay in the right block.

As you can see, there is virtually no rink to end up with a dude for measurememtns anywhere in area #1, but area #2 contains botht the smallest HCA scores (and some would say that less in not always better, but all above 2 is great) and a small green area - meaning a range of combinations of crown and pavilion angles that do not promise as much as the rest, or a small risk to end up with something less than perfect.

If the table percentage varies, the chart would change slightly, so the actual limits for desirable crown angles (or the tollenrance for pavilion angle variation) changes a bit - and this is what those two big balck arrows try to say.

Just my 0.2, as always

 

[valeria101]

Just one example, (hoping i got the Q right!):

The range on angles and measures are taken for teh Sarin report of a stone with some H&A showing, but not perfect ones (actually, quite mangled, relative to what those ACA and 8* show), three VH on the BScope and a rather red Iscope pic, but with a few white areas produced by the so-so symmtry

To get teh ballpark color code for this stone on the HCA chart, one would have to mix all that is inside those little areas where the measurements vary ! (this did not take into acount Sarin's measurememnt error, which I could not find in 10 min). It would not be "correct" to say that "this stone has some chance to fall into the bad part of the area" - 'cause all those measurements are actually valid for some portions of the stone.

I would rather ake an analogy with the Iscope picture and say: well, most of it is great, but maube some small areas would not be deep red, but show moderate (read insignificant, and there is a thread on what those 'pinks' mean, somewhere) light loss. And since the worst this range does is still great (orange on the chart is the fabled HCA=2 actually), what this ball-parking says is exactly what the other tests of brilliance do - (in this case).

Hope this makes sense and helps, it surely was fun play for me

 

[valeria101]

EVER, EVER wandered what that H&A pattern is good for ?

Well, it does offer and elegant way to survey the symmetry (and proportions actually) of the cut, and reduces that range of variation pictured above to NADA

In the picture below the small blue segment shows the "area" of probable HCA variation for a stone with top H&A symmetry ( consistent with Isee2 9.. The Brilliance Scoe scores were also three VH, as for the near H&A above. Of course, no area of light loss on the Light Scope picture (aside what the H&A pattern allows - meaning some specs near girdle), just as the range of measurement variation does not touch outside the 'dream zone' (red) below

 

[strmrdr]

The first thing I look for in a diamond is how tight the demensions and angles are on the sarin while it doesnt cover enough facets to make a decision on if the diamond is cut right it is an indicator that the rest may be equaly well cut.
Comparing the "tightness" on the sarin to the h&a and scope images is an interesting exercise.
If one facet is off it will usualy show in the images and it is fun to track it down and compare the measurements to the images.

Even more fun is to start off with an almost h&a image and trace back to the sarin report to figure out why its almost h&a and what facet is responsible for the "defect" in the image :}

Even more challenging is take a b-scope triple vh diamond compared to say a 2h 1vh diamond and try and figure out the differences based on the sarin data.

When doing all this you have to keep in mind however that the sarin has a margin of errer so its never a precise exercise but you can often track down the larger differences. :}

 

[valeria101]

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On 5/2/2004 11:29:57 AM strmrdr wrote:




If one facet is off it will usualy show in the images and it is fun to track it down and compare the measurements to the images.

Even more fun is to start off with an almost h&a image and trace back to the sarin report to figure out why its almost h&a and what facet is responsible for the 'defect' in the image :}

Even more challenging is take a b-scope triple vh diamond compared to say a 2h 1vh diamond and try and figure out the differences based on the sarin data.

When doing all this you have to keep in mind however that the sarin has a margin of errer so its never a precise exercise but you can often track down the larger differences. :}
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I know it's fun! What do you think I've been killing time with lately

I just used the Iscope 'testimony' as a check on Sarin results: the machine might have it's (as yet unknown to me) margin of error, but the handling of it is yet another issue. Just ask "emeraldseeker"!

If you look at the "Manufacturer" version on those Sarin reports you can get an idea what the distribution of (measured) error is for each stone...


However, to assemble a reasonable sample I'd need a second life

not just one funny looking Excel chart. And I should kick this PS addiction anyway!

 

[strmrdr]

Val,
I think we are in the same boat.
The more one learns the more questions come up.
I have a hard time accepting that it isnt yet an exact science.
I honestly dont think it will ever be and thats even harder for me to accept.
When I think I have it all down pat the exception will come along or my data set will be too small and my conclusions will get blown out of the water.
It does make it harder to provide honest answers to peoples questions when you question your own understanding of the data provided.

 

[Rhino]

If I may jump in...




If a diamond has average pavilion angles of 41 degrees you guys are right in seeing how many of those pavilion angles are OVER 41 degrees, how many are UNDER, and how many that are right at 41 degrees. A teter just over or under the 41 degree mark can make or break a buying decision for us.




If most of the pavilion facets fall over the 41 degree zone you will see the direct results of this in an optical exam. Particularly B'scope results and to a lesser degree Isee2 results. The question that begs an answer here is *what crown angles are coupled with those pavilion angles that do fall greater than 41 degrees*.




When I am inquiring for diamonds from vendors and they tell me average pavilion angles of 41 degrees these are the questions that run through my head. They can either be winners or losers and I feel safer if those pavilion angles fall within the spectrum of 40.7-40.9. At the end of the day it is the optical results that count most so if you see a stone with 41 degree pavilion angles on average it helps to know the results of critical testing.