Calculating the pavilion angle

[Deia]

I felt bad when I saw this person didn't get a reply so am posting it here for him/her to get more answers.

By the way never noticed we had a Diamond Reasearch forum until just a few minutes ago!

Hi All,

I wanted to ask a very technical question that only the most qualified can answer definitively. I deciding whether or not to buy a round brilliant stone that was GIA certified in 2000 and unfortunately they didn't measure the Pavilion Angle only the Crown Angle; nor did they assign a cut grade during that time. The specs of the stone are below:

Round Brilliant
Measurements: 6.27 - 6.32 x 3.79 mm
Carat Weight: 0.91 carat
Color Grade: F
Clarity Grade: VS2

Proportions:
Depth: 60.2%
Table: 60%
Crown Angle: 33.0
Pavilion Angle: ??
Girdle: Thin to Medium, Faceted
Culet: None

Finish:
Polish: Good
Symmetry: Good
Fluorescence: None

Can the other missing dimensions/angles be calculated based on the above information? I'm trying to determine the cut grade based on the dimensions of the stone. I saw an article here about the myth of 60/60 which this diamond falls into so I wanted it to ask some experts if this could be a very good to excellent cut stone.

Second question, given all the information what is a fair price for the stone?

I hope I didn’t overstep the boundaries of this forum and I appreciate any advice technical experts can give.

 

[kenny]

My math skills are now too rusty to do it but I'm pretty sure it is possible to solve for pavilion angle with the given numbers BUT if you wanted to be super precise you'd need to know the thickness of the girdle in mm, not just "thin to medium".

 

[stone-cold11]

With good symm, might not even be applicable with cylindrical symmetry. Numbers may not be useful for HCA.

Just view it under an idealscope.

 

[yssie]

With good symm, might not even be applicable with cylindrical symmetry. Numbers may not be useful for HCA.

Just view it under an idealscope.

ditto that.
1. I thought older GIA had neither crown nor pav info, where is that coming from?
2. w/ poor physical symmetry having averaged, rounded pav/crown info doesn't tell you much about how the stone will actually look (what sorts of ranges of values are going into those averaged rounded numbers on the report)
3. crown height would be tan33deg*[(avgDiam-60% of avgDiam)/2] ~13%
http://www.gia.edu/diamondcut/pdf/poster_finish_culet_girdle_lowres.pdf assume for convenience avg 3.2%, pav depth is 60.2-13-3.2~44% = arctan(44% of avgDiam/half of avgDiam) ~41.4deg
As SC says it doesn't actually mean anything since we don't know anything about avg girdle thickness and overall girdle variance, lower girdle facet length (and therefore angle), brillianteering... long story short - if your eyes like it, and it's chump change for you, buy it! If not, then don't. This isn't one to pick by the numbers.