GIA rounding - how exactly does it work?

[scramblezy]

Have noticed that this comes up a lot as an issue with GIA certificates - the gross rounding of proportions as compared to labs like AGS. I ventured onto the GIA website but wasn’t able to fully understand how they do their rounding for borderline cases and was wondering if people here would be able to answer that?

Table percentage: rounded to the closest 1%. Does this mean for 56.4% and below it gets rounded to 56%? What about say 56.5%? Does it get rounded to 56% or 57%?

Crown angle: rounded to the nearest 0.5 degrees. Again, is it rounded down to 34.0 if it’s 34.4 and below? And then to 35.0 if it’s 34.6 and above? Or is it always rounded up to the next 0.5 regardless?

Pavilion angle: rounded to the nearest 0.2 degrees. Is it always rounded up?

Just hoping to better understand the possible actual angles when viewing GIA reports. Thank you!

 

[sledge]

Regardless of AGS or GIA, in a round diamond, you need to understand each stone has 57 (or 58 if counting the culet) facets:

• 1 table facet
• 8 crown facets
• 8 star facets
• 16 upper half facets
• 8 pavilion facets
• 16 lower half facets
• 1 culet (although on modern round stones this is typically pointed, or 0)

Yet when you see a lab report, rather GIA or AGS, you will simply see a single value reported for each respective proportion (table, crown angle, pavilion angle, etc). Some vendors such as WF, IDJ, B2C, etc may offer a detailed SARIN report that includes the values of all the facets. Usually these are provided upon specific request assuming the vendor has the ability to provide them.

The above noted, to get from 8 crown angles or 16 lower girdle facet values averaging occurs with both laboratories. Once averaged AGS rounds to the nearest 0.10 degree or percent, except the LGF's and stars which are rounded to the nearest 1%.

GIA rounds differently as noted below:

• Nearest 1% for table
• Nearest 0.5 degree for crown (ie 34.5 may include 34.3-34.7)
• Nearest 0.2 degree for pavilion (ie 41.2 may include 41.11-41.30)
• Nearest 1% for LGF's (ie 75 may includes 73-77)
• Nearest 1% for stars

So as you can see, we condense from 8-16 actual values down to a single value and then we round those to a tolerance each lab has established. The real unknown kicker is unless you have that detailed SARIN report, you don't really know what compiled the average in the first place. More importantly we don't know how the actual of a particular individual crown angle may work in relationship to it's corresponding individual pavilion angle creating further uncertainty.

For all these reasons, I prefer AGS when possible. Also with the AGS platinum report, you get a ray-traced ASET image, so if the vendor in questions doesn't have advanced images, you have that at least. With GIA I try to be a little more conservative about what may or may not work. But getting advanced images & video really aides the process in selecting a stone. Obviously seeing with your eyes is always a bonus as well but not always an option for online buyers.

I am sure some of the trade guys like @John Pollard, @Karl_K or @Garry H (Cut Nut) can jump in and expand further. But I think this is a fairly decent basic understanding.

Reference articles:

Diamond Cut: Anatomy of a Round Brilliant

Jewelers and gemologists use a specific set of terms to describe parts of a polished diamond. Certain measurements of the shape or proportions ultimately have an affect on the round brilliant diamond’s cut grade.

www.gia.edu

https://www.gia.edu/doc/booklet_cut_estimation_tables_lowres.pdf

 

[John Pollard]

@sledge - you crushed that overview.

In summary -

1. A round diamond's 57 ( or 58 ) facets are measured using a 3D non-contact scanner. You can see photos and a description here.

2. This -

So as you can see, we condense from 8-16 actual values down to a single value and then we round those to a tolerance each lab has established. The real unknown kicker is unless you have that detailed SARIN report, you don't really know what compiled the average in the first place. More importantly we don't know how the actual of a particular individual crown angle may work in relationship to it's corresponding individual pavilion angle creating further uncertainty.

3. Here are the numbers of measurements any given data point represents, and how AGS reports those averages, as opposed to most others.

Click here to see the chart it at a higher resolution.


And, in case it's useful for future readers, here's where to find key proportions on AGS and GIA grading reports.



Read more on our 4Cs overview covering Diamond Cut.

Diamond Cut: Everything You Need To Know

What's the right diamond grade for your budget? Read our extensive guide to diamond cut and how it can impact your diamond's sparkle.

www.pricescope.com

 

[sledge]

Thanks @John Pollard.

Re-reading I saw my initial post had a typo and I am beyond the time limit to correct. Both GIA stars and LGF’s are rounded to nearest 5%, not 1%.

In my example of LGF actual values (73-77) for a 75 reported value, it’s probably obvious what was meant. Still, sorry if it confused anyone.

 

[denverappraiser]

FWIW, they have an unusual system for rounding the weight. It's the standard rules we all learned in school but with some special added rules around the integral carat weights. 1.999 will 'round' to 1.99. 1.985 will also be reported as 1.99. 2.005 = 2.01.

 

[scramblezy]

So much information - thank you! Much clearer now and I can see why AGS reports especially the platinum ones are preferred!

 

[CutMonkey]

FWIW, they have an unusual system for rounding the weight. It's the standard rules we all learned in school but with some special added rules around the integral carat weights. 1.999 will 'round' to 1.99. 1.985 will also be reported as 1.99. 2.005 = 2.01.

My understanding is that carat weight is only rounded up to the nearest 100th if the 1000th digit is a 9.

Thus:

1.981 is reported as 1.981, or rounded to 1.98
1.985 is reported as 1.985, or rounded to 1.98
1.998 is reported as 1.998, or rounded to 1.99
1.999 is reported as 1.999, or rounded to 2.00
2.008 is reported as 2.008, or rounded to 2.00

How is diamond carat weight rounded?

Diamonds are weighed to the thousandth (.001) of a carat (ct) and then rounded to the nearest hundredth (.01). The weight will only be rounded up to the nearest hundredth if the thousandth digit is a nine. For example, a stone that weighs 1.768 ct would be rounded to 1.76 ct, but one that weighs...

www.gia.edu

 

[John Pollard]

My understanding is that carat weight is only rounded up to the nearest 100th if the 1000th digit is a 9.

@CutMonkey , you're right. That's current practice at GIA.

It might be interesting to know that policy changed at some point. This snap is from my 2006 GIA DG Lab Manual (please open your hymnals to page 135).



@denverappraiser , I know several other labs changed to match GIA's policy. Do you know if AGS continues to use conventional rounding? If so I'd like to add that distinction to our EDU page: Relevant section can be found here.