Pavilion Depth to Pavilion Angle in GIA report

[defy525]

Hi folks,

I have found two posts which show the way to calculate depth % to angle:
https://www.pricescope.com/communit...-calculating-crown-and-pavilion-angles.20396/
https://www.pricescope.com/community/threads/percentages-to-angles-using-trig-calculator.13404/

From my understanding, GIA report rounds up the Pavilion Depth to nearest 0.5%, is it right?
Using the chart below:

The Pavilion Angle is 40.6 degree and I try to calculate the Pavilion Depth with the equation:
tan(40.6 degree) x 50 = 42.855%

Is the actual Pavilion depth to be 42.855%? The GIA report shows a 42.5% instead, is there any other factor affecting the equation, like girdle thickness?

At last, what do you think about the proportions of this diamond?

Thanks a lot!
Defy

 

[defy525]

Just had a brief search on other diamonds, it seems that most of the diamonds with 40.6 degree pavilion angle in GIA report pair with 43% pavilion depth in general. Only a few pair with 42.5% pavilion depth.

 

[gm89uk]

Hi defy525. Few factors here:
Your equation does not take culet into account 50((1-culet size%)x tan(pavilionAngle)). As culet size increases your pavilion depth decreases. Culet is graded at x10 loup and very very small culets can affect which way GIA rounds the pavilion percentage. As you say 40.6 equates to 42.855% which is rounded up to 43%. If the culet was 0.03% (graded as none on the report) it would come to 42.727% which would be rounded down to 42.5%.

However the main contributor is that GIA rounds the pavilion angle as such that the decimal point is always even. Odd numbers are rounded up to the nearest even decimal point
1) GIA Rounds. So 40.6 can mean anything between 40.5 to 40.69999etc.
40.5 will read 40.6 on the report which will be 42.704% (rounded down to 43).

So you either have a very very small culet not measurable at x10 loup or an angle thats slightly less than 40.6 and rounded up.

I wouldn't worry about it all too much. When looking at a stone with 34/40.6 they are both on the shallower end of ideal range. This can lead to some obstruction on close up viewing and such angles are very well suited to earrings and pendants but sometimes less suited to ERings. They can be gorgeous stones but just make sure it doesn't look dark on close viewing.

 

[defy525]

Thanks gm89uk!! you are pro I always think the culet is 0% and forget it is also rounded.
The stone is for ERing, the crown view of the stone is shown in my profile picture.