Is there an AGS calculator?

[mission1]

I’ve found various tools which assess and report on likely diamond performance based on input parameters, but I wondered if there are any tools that would generate a likely AGS grading? I have a GIA stone and would be interested in what its AGS equivalency is - all I’ve found is the GIA exc / AGS Ideal plot map.

 

[mission1]

Actually, Ianswering my own question, I've just found this on the PS wiki: https://www.pricescope.com/wiki/diamonds/diamond-grading-chart-round

I think the cut of the diamond I've bought falls into 1A except for pavilion depth (0.3 off the lower requirement for 1A):

Table: 56% - 1A
Crown angle: 34% - 1A
Crown height: 14.5% - 1A
Pav depth: 42.5% - 1B
Girdle: T-M 3% - 1A
Depth: 60.2% - 1A
Pol/Sym: Ex/Ex - 1A

 

[denverappraiser]

That's AGA, not AGS.

In answer to your question, yes there are ways to estimate an AGS grade, but they're not free. Last I checked, the AGS software license was $1000/year but I'm sure they'll talk to you about it if you like. Give 'em a call.

 

[Texas Leaguer]

With the advent of the AGS light performance cut grading system that actually ray traces a 3D scan of the diamond, the older table-based system has been made much less relevant.

Tools like the GIA Facetware and HCA use a much less sophisticated 2D system that looks up predefined grades on a table.

Having said that, you can compare the data you have (remember it is averaged and rounded) to the appropriate table at the link below to get an idea where it might fall within the AGS system.

Round Brilliant Cut | Diamond Cutting Chart Guidelines - American Gem Society

Check out all of the round brilliant cut diamond cutting charts at AmericanGemSociety.org today.

www.americangemsociety.org

 

[mission1]

Thanks - was just idle curiosity really!

 

[sledge]

I'm guessing the stone you are asking about has a 40.6 paviion angle, resulting in the 42.5 depth.

Mathematically the depth should calculate to: tan(40.6) * 50 = 42.855, which would be rounded to 43.

One of two scenarios exist. The culet is not exactly 0, or none, and you have to modify the formula for that variant. Even a slight 0.03% can rock the boat as evidenced below:

[tan(40.6) * (1.00 - 0.003)] * 50 = 42.726, which would then be rounded down to 42.5%

Alternatively the actual pavilion angle could be slightly less than 40.6 and because of the way GIA rounds and averages, it got reported as a 40.6.

tan(40.5)*50 = 42.704, which would be rounded down to 42.5 on the report

I built a spreadsheet using the old AGS proportions charts from 2008; however, I probably need to update the chart data with the 2017 data posted by @Texas Leaguer (thank you!).

This would still be very elementary compared to the 3D scans mentioned. But it may give a rough approximation. Here is a sample of the one I built.

 

[mission1]

Hi Sledge, that’s really interesting thanks. Yes, according to the GIA cert it’s a 40.6 pavilion angle and no culet. So it looks like it’s well in the AGS excellent rather than Ideal based on your chart and the info above.

I did a lot of research before choosing it online, but I must admit that looking at a tiny little stone in my fingers it’s hard to know whether I made good choices! Anyway, it’s being set imminently so that’ll take away my opportunity to ponder... hopefully she’ll really like it (engagement ring) and it’s a better stone than I may’ve got had I just gone to a jeweller and not done my research!

 

[mission1]

On that last point, link below is for an IGI cert on a diamond that a jeweller wanted to sell me - he was someone recommended by a friend who used him. He said that it was an amazing diamond with stunning fire, and he was offering it for around 15% more than the one I got online. Didn’t look like a great diamond..... https://www.igi.org/viewpdf.php?r=337817146

 

[sledge]

Hi Sledge, that’s really interesting thanks. Yes, according to the GIA cert it’s a 40.6 pavilion angle and no culet. So it looks like it’s well in the AGS excellent rather than Ideal based on your chart and the info above.

I did a lot of research before choosing it online, but I must admit that looking at a tiny little stone in my fingers it’s hard to know whether I made good choices! Anyway, it’s being set imminently so that’ll take away my opportunity to ponder... hopefully she’ll really like it (engagement ring) and it’s a better stone than I may’ve got had I just gone to a jeweller and not done my research!

To me, it sounds like you made a pretty solid selection. I don't have all your data on the stone, but based on what I've seen on table, depth, crown & pavilion you should run it through the HCA calculator to see what score you get. Anything below 2 blows people's skirts up. Based on your proportions I think you will be around 1 or less.

The thing with choosing stones based on proportions is the data we see on the lab report is compiled from 8 actual crown and pavilion angles. That data is then averaged and rounded by GIA so you see just a single crown & pavilion value. That means there is some variance.

Then you take the HCA and it uses the lab data to make further predictions.

I think you see where I'm going. You average & round numbers then HCA makes it own assumptions/predictions on that averaged & rounded data and it gets us close but it's not always exact. That's where either advanced imaging and/or our actual eyes confirms it's beauty. It sounds like you saw this stone, so I'd rest easy in your selection and start prepping for the big proposal.

Congrats on picking a beautiful stone, and the soon to be engagement!

On that last point, link below is for an IGI cert on a diamond that a jeweller wanted to sell me - he was someone recommended by a friend who used him. He said that it was an amazing diamond with stunning fire, and he was offering it for around 15% more than the one I got online. Didn’t look like a great diamond..... https://www.igi.org/viewpdf.php?r=337817146

Yikes....IMO, you avoided a bullet my friend.

IGI USA labs get a pretty bad rep. Supposedly some of their other labs in global markets are more reliable and trustworthy. This cert appears to be from Antwerp so it may have more validity and respect. I am not super familiar with them so I will defer to the more experienced to comment on that aspect, if they so wish.

Either way, if we just assume the cert is accurate, this is not a stone I would have recommended for you.

While not always indicative of anything, a stone with an exact 1.00 carat weight makes me automatically think it was cut to hit a magic weight.

And as we look at the proportions, this is confirmed. It's a steep & deep stone -- 58.5 table, 64.2 depth, 35.7 crown & 41.7 pavilion. Additionally, the symmetry is only very good, as opposed to excellent.

Lastly, the stone is an SI2 with some decent sized clouds, so I would be concerned if the stone would have a hazy/cloudy appearance.

For grins, I'm attaching an HCA score on this stone and also a screen cap of my Excel predictor.






Keep in mind on my calculator it's trying to input GIA values, which only accept whole numbers on table sizes (58 or 59, but not 58.5) and crown angles are entered in the nearest 0.5 degrees (35.5 or 36, but not 35.7). But it gives you a good idea. It's in territory you never want to be in, lol.

 

[mission1]

Thanks for the reassurance Sledge. My stone gave an HCA score of 0.4. I was slightly worried about it being dark, but it doesn't seem to be (and there wasn't much in the way of obstructions showing in the images).

The IGI stone seemed bad even to my rudimentary knowledge (though much aided by resources on PS) - with its depth in particular jumping out. He described it as "beautiful and full of life. Amazing fire. eye clean". Although it's a 1 carat, it faced up similarly to the 0.9 that I went for.

 

[sledge]

LOL, sounds like that jeweler is the type of guy that lists a car as perfect condition with minor fender bender damage, and posts no pictures. Then you go to see the great deal and you encounter this: