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averaging and rounding

slg47

Ideal_Rock
Joined
Apr 4, 2010
Messages
9,667
I am interesting in the differences in reporting numbers between AGS and GIA. GIA averages the numbers and then rounds (listed below). I could not find the description of how AGS reports numbers but I know they do not round crown to the nearest 0.5.

My question is, why the difference in how the numbers are reported? I have quoted below from the GIA website.

The GIA Diamond Cut Grading System and the GIA Facetware® Cut Estimator use averaged, rounded values for proportions when assessing diamond cut quality.

The rounding used by the system is as follows:

Total depth is rounded to the nearest multiple of 0.1%
Table size is rounded to the nearest multiple of 1%
Crown angle is rounded to the nearest multiple of 0.5°
Pavilion angle is rounded to the nearest multiple of 0.2°
Pavilion angles ending in odd numbers are always rounded UP, for example 40.7° would be rounded to 40.8°
Star length is rounded to the nearest multiple of 5%
Lower-half (lower-girdle facet) length is rounded to the nearest multiple of 5%

Measurement precision and rounding was determined by considering:

The precision of measuring devices and other measuring methodologies
The practical abilities of most diamond cutters and manufacturers
The differences in cut quality that were found to be consistently distinguishable during our extensive observation testing

These three considerations work together, so we used the highest level of precision that could be applied in all three categories.
 

John P

Ideal_Rock
Trade
Joined
May 1, 2008
Messages
3,563
The largest difference is that GIA's assessment is general whereas AGSL's is diamond-specific.

What this means is that GIA takes a diamond's averaged/rounded data points and correlates them to general lookup-charts.
AGSL scans each diamond and conducts ray-tracing on that specific scan to assess how all 57(58) facets work together.

As far as reporting, AGSL gives averages to the nearest 0.1° angular and 1% linear. As mentioned though, the evaluation involves far more than just those data points. The ray-trace gives brightness, contrast, leakage and basic dispersion assessments through two different levels of obstruction as well as a range of tilt. The grade on the report was "earned" by that specific diamond, not assigned based on rounded averages.

With that said, neither lab is yet meaningfully assessing dispersion or scintillation. While both systems were developed through solid research they are largely confined to brightness, contrast and leakage assessments only.
 

affguy

Rough_Rock
Joined
Feb 18, 2011
Messages
100
I'm neither GIA nor AGS, but when people ask about my fiancee's .968 AVC with four 1-pt melee in the setting, well, I tell them it's a carat. :naughty:
 

Regular Guy

Ideal_Rock
Joined
Jul 6, 2004
Messages
5,954
slg47|1316549136|3021563 said:
My question is, why the difference in how the numbers are reported? I have quoted below from the GIA website.

The GIA Diamond Cut Grading System and the GIA Facetware® Cut Estimator use averaged, rounded values for proportions when assessing diamond cut quality.

Fundamentally, JP's answer may be the most helpful, overall.

But, I'm betting the answer to your question, SLG, lies in the note below. Really, I think the best contributions I made to Pricescope were in my first year here, now 6 years ago...

https://www.pricescope.com/communit...-a-round-diamond.36869/?hilit=proportionscope

The suggestion is that standards of reporting from GIA have been made consistent with the equipment that GIA would require its appraisers to articulate with...i.e., thus making sense of the lowest common denominator available in the equation.

I think other threads embrace this sort of speculation as well...


Ira Z.
 
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