Pavilion Angle and Pavilion Depth don't perfectly correlate - why?

[Tiff1886]

One thing I have noticed after viewing a couple hundred Gemological Reports from Tiffany:

The Pavilion Angle and the Pavilion Depth do not always perfectly correlate.

For a given pavilion angle (example: 40.9), the pavilion depth varies from 43.1 to 43.4. (I even found a case where a 40.8 PA has a 43.2 PD while a 40.9 has a 43.1 PD.)

I imagine this is all down to rounding because there can be quite a bit of variance in the individual pavilion mains?

If one were to pick a 40.9 with a 43.2 depth percentage, does that mean the pavilion is on the shallower end of 40.9?

I attached some random examples from eBay to show the discrepancy in Pavilion Angle vs Pavilion Depth

 

[yssie]

One thing I have noticed after viewing a couple hundred Gemological Reports from Tiffany:

The Pavilion Angle and the Pavilion Depth do not always perfectly correlate.

For a given pavilion angle (example: 40.9), the pavilion depth varies from 43.1 to 43.4. (I even found a case where a 40.8 PA has a 43.2 PD while a 40.9 has a 43.1 PD.)

I imagine this is all down to rounding because there can be quite a bit of variance in the individual pavilion mains?

If one were to pick a 40.9 with a 43.2 depth percentage, does that mean the pavilion is on the shallower end of 40.9?

I attached some random examples from eBay to show the discrepancy in Pavilion Angle vs Pavilion Depth

I don’t know if Tiffany and GIA measure & calculate the exact same ways. Presuming they do,

The pavilion angle is measured like this:
1. Measure the angle made between table plane and one pavilion main.

2. Repeat (1) for the other seven mains.
3. Take the average of all eight angle measurements.

The pavilion depth is calculated like this:
1. Measure the diameter of the stone (girdle diameter) where one pair of opposing pavilion mains meet the girdle.

2. Repeat (1) for girdle diameter measurement between the other three opposing pavilion main pairs.
3. Calculate the average girdle diameter.
4. Measure the distance (“height”) between where one pavilion main meets the girdle and the plane tangent to the culet.

5. Repeat (4) for all other seven pavilion mains.
6. Calculate the average pavilion “height”.
7. Calculate the pavilion depth percentage as (average pavilion “height” / average girdle diameter).


So pavilion depth percentage is a calculation of two other (averaged) measurements. No matter what precision you originally measure girdle diameter and pav “height” to, the fact that you’re then calculating something based off averages of those measurements aggregates both the error of those measurements and the error introduced by averaging. Pavilion depth is ALWAYS going to be less trustworthy than the directly measured (albeit also averaged) pavilion angle.

Re. Pavilion angle measurement - the table plane and the girdle plane should be parallel. If they’re offset a teeny tiny bit you won’t notice outside of through a H&A viewer. If table is skewed to any meaningful degree - it’d be visible in photos, in the IS, and most likely a symmetry grade demerit as well.

Takeaway: When you’ve got pavilion angle, you can ignore pavilion depth entirely. Def no need to worry about minute percentage differences. Remember that pavilion depth is actually a percentage, they just leave the percent signs off on the report, so a diff of 43.1 vs 43.3 isn’t a “tenth” unit difference (like say with pavilion angle) it’s “thousandths”.

 

[tyty333]

@Tiff1886 I really think you should keep all this Tiffany info in one thread to make it easier for future Tiffany researchers to find
"all" of it at once. Unfortunately, this thread doesn't have the word Tiffany in the title so it may be harder for Tiffany researchers to find...plus others may think that it applies to GIA.

So, you might ask Admin to combine the threads and put it under one good title with "Tiffany" in it.

 

[Karl_K]

Everyone uses non-contact scanners and the angles are measured and the depth % is calculated.
Also all the numbers are rounded so a .01% actual difference can look like a .1% difference on the report.
Girdle variation explains larger differences.

 

[Tiff1886]

@Tiff1886 I really think you should keep all this Tiffany info in one thread to make it easier for future Tiffany researchers to find
"all" of it at once. Unfortunately, this thread doesn't have the word Tiffany in the title so it may be harder for Tiffany researchers to find...plus others may think that it applies to GIA.

So, you might ask Admin to combine the threads and put it under one good title with "Tiffany" in it.

I’m happy with whatever best serves the forum. We can combine if that is better.

I originally made this a separate thread just because I figured a deeper understanding of pavilion angle vs pavilion depth would be of benefit to anyone looking at any type of report. I know GIA rounds off pavilion angle to the nearest 0.2 degree and pavilion depth % to the nearest 0.5 percentage point - so for those looking at GIA, they will not have the same resolution.

Thanks to the responses so far I now see that the pavilion angle is the more useful measurement. I appreciate you all taking the time to explain this once more.

 

[Tiff1886]

@yssie
Your explanation is excellent. Thank you.

 

[headlight]

I’m happy with whatever best serves the forum. We can combine if that is better.

I originally made this a separate thread just because I figured a deeper understanding of pavilion angle vs pavilion depth would be of benefit to anyone looking at any type of report. I know GIA rounds off pavilion angle to the nearest 0.2 degree and pavilion depth % to the nearest 0.5 percentage point - so for those looking at GIA, they will not have the same resolution.

Thanks to the responses so far I now see that the pavilion angle is the more useful measurement. I appreciate you all taking the time to explain this once more.

I personally appreciate the diffeeent threads as I do see them as different discussions.

 

[Garry H (Cut Nut)]

Angles are what cause light to deflect, refract and reflect.
%s are a foreign language for light.
A small culet can make a difference to the depth %