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Why is this diamond a GIA "Very Good" rather than "Excellent"?

Nafrenchfry

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Hello!

I am brand new to the diamond community and am just starting to learn the basics. I've tried to educate myself as much as possible but am confused about the cut grading on a diamond I just bought for an engagement ring. (So, yes, my search is over but I'm still just curious about learning more about the industry and how it works.)

The diamond I purchased has a "Very Good" cut but, based on my very elementary understanding of things, it appears to have better proportions than some of the "Excellent" diamonds I looked at. So, my question is, why is this diamond evaluated as a "Very Good"? What is the effect of whichever proportion is not "Excellent"? In other words, am I going to regret not buying an "Excellent" cut diamond? (In case you're curious, it's a Round 0.8 ct E VS2.)

Polish: Excellent
Symmetry: Excellent
L/W Ratio: 1.01
Depth %: 62.30
Girdle: Medium-Slightly Thick
Table %: 56.00
Culet: None
Crown Angle: 36.00
Crown %: 16.00
Pavillion Angle: 40.60
Pavillion %: 42.50
Star length %: 55
Lower half %: 80

It appears to have an HCA score of 1.8 and an AGS cut grade of 1B. I don't have any information on it regarding IdealScope.

Thanks, in advance, for helping me continue my education into the world of diamonds. :)
 

yssie

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Post the report.
I want to know who evaluated it and see the proportions diagram.
Are there any comments regarding brillianteering?
 

skypie

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The crown angle seems too high, diamond may be a touch deep too.
 

Nafrenchfry

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Gotcha. A couple places I've seen said that the upper limit for crown angle for an excellent cut diamond is 35.8 so I didn't think that 0.2 degrees would make that much of a difference but I guess it might be enough to downgrade it. Still, do you think I made a good choice in diamond I purchased in terms of its brilliance and fire? I guess there's always the 30 day return policy but I'm getting the sense my girlfriend is getting impatient so not sure how much longer I should hold off on a proposal. :lol:
 

skypie

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Gotcha. A couple places I've seen said that the upper limit for crown angle for an excellent cut diamond is 35.8 so I didn't think that 0.2 degrees would make that much of a difference but I guess it might be enough to downgrade it. Still, do you think I made a good choice in diamond I purchased in terms of its brilliance and fire? I guess there's always the 30 day return policy but I'm getting the sense my girlfriend is getting impatient so not sure how much longer I should hold off on a proposal. :lol:

Due to rounding it could be more than 0.2 degrees difference, but yes even a 0.2 degree difference can be significant.

Really no way to evaluate performance without seeing it in person and seeing ASET/IS images.
 

blueMA

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Gotcha. A couple places I've seen said that the upper limit for crown angle for an excellent cut diamond is 35.8 so I didn't think that 0.2 degrees would make that much of a difference but I guess it might be enough to downgrade it. Still, do you think I made a good choice in diamond I purchased in terms of its brilliance and fire? I guess there's always the 30 day return policy but I'm getting the sense my girlfriend is getting impatient so not sure how much longer I should hold off on a proposal. :lol:

The VG is for the overall cut grade, not necessarily for the stone's angles, at least under the GIA. The AGS is more strict, but GIA still allows 36/40.6 combo under EX unless other cut related issues are present.

upload_2019-1-18_12-28-23.png
https://www.gia.edu/doc/booklet_cut_estimation_tables_lowres.pdf
 

Kaycee2018

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I'm no expert, but it isn't the crown angle. I have a 36 deg. Crown Angle paired with a 40.6 deg. Pavilion Angle and mine is a GIA XXX and I have seen many, many others with that same CA/PA combo that are GIA XXX.
 

blueMA

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Still, do you think I made a good choice in diamond I purchased in terms of its brilliance and fire?
The diamond is not what's considered as a balanced stone, but it can be a very fiery (lots of colored light vs white light) stone. Without a photo/video or ASET/Idescope image, there's no telling how this diamond would perform.
 

sledge

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Do you have a link to the webpage where you bought it? Sometimes they will have videos and pictures, which may be helpful.

But honestly, I think you are dwindling down a rabbit hole of "what if's". Take the mystery out of the equation. Spend $50 and buy an ASET scope. Take a picture and post it.

Then we can give you some valuable feedback. Also, if it the ASET doesn't show favorable results, it allows you to return the stone within your 30 day window and trade out for a better stone. While a hassle, it's really a blessing IMO as it gives you an opportunity to fix a potential problem and alleviate your concerns with empirical evidence.
 

yssie

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GIA facetware agrees with the VG: https://www.gia.edu/facetware
The table referenced by @blueMA above uses a default girdle thickness of 3.0%.

Reducing girdle (to 3.5%), reducing crown (to 35.5%), and increasing table (to 57%) all serve to both reduce total depth and increase potential score to EX.

My honest opinion: You've bought the stone, right? So stop worrying over it - all women know that a tad extra at the waistline never hurt anyone ::) There are a handful of GIA VGs that I would choose over most GIA EXs - this crown/pav/table/lgf combo is one of them. Assuming optical symmetry is decent, which is where a good photo, or ASET, or IS, or H&A image would help.
 

sledge

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GIA facetware agrees with the VG: https://www.gia.edu/facetware
The table referenced by @blueMA above uses a default girdle thickness of 3.0%.

Reducing girdle (to 3.5%), reducing crown (to 35.5%), and increasing table (to 57%) all serve to both reduce total depth and increase potential score to EX.

My honest opinion: You've bought the stone, right? So stop worrying over it - all women know that a tad extra at the waistline never hurt anyone ::) There are a handful of GIA VGs that I would choose over most GIA EXs - this crown/pav/table/lgf combo is one of them. Assuming optical symmetry is decent, which is where a good photo, or ASET, or IS, or H&A image would help.

@yssie, when running GIA facetware and entering this data why does it kick back a pavilion & overall depth that disagree with the cert? In this case it says pavilion is 42.6 (vs 42.5) and overall is 62.6 (vs 62.3).

Just wondering if we are teter-totering between VG and EX, if facetware kicking back values that are slightly different would force it one way or the other?

Not disputing any information -- trying to learn. Appreciate if you can explain more. Thanks.

Screenshot_20190118-132232_GIA Facetware.jpg
 

gm89uk

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@yssie, when running GIA facetware and entering this data why does it kick back a pavilion & overall depth that disagree with the cert? In this case it says pavilion is 42.6 (vs 42.5) and overall is 62.6 (vs 62.3).

Just wondering if we are teter-totering between VG and EX, if facetware kicking back values that are slightly different would force it one way or the other?

Not disputing any information -- trying to learn. Appreciate if you can explain more. Thanks.

Screenshot_20190118-132232_GIA Facetware.jpg

Hey Sledge,

GIA Facetware is to acquire total depth is adding GH + PH + CH.

If table is exactly 56%, with exact 36degree crown, then CH is 15.98% or 16% (tan36)(1-0.56)/2
If culet is pointed pavilion angle is exactly 40.6, then PH is 42.86% (tan40.6/2)
If the girdle is exactly 4% then in total it's 4 +15.98 + 42.86 = 62.84% depth.

GIA Facetware is rounding CH to 16, PH to 42.6 and girdle to 4 to it gets to 62.6% total.

In reality some of these values are rounded up, hence why the actual depth percentage is less.

A diamond with those proportions the chances of the pavilion < 40.6 are high, and crown less than 36 are high.

If the crown height is 35.9, and pavilion 40.55, then that'll make up for the discrepancy in depth %.

Still not sure why VG. Maybe brillianteering of the diamond.
 
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Karl_K

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Sounds like a gia VG that will kick a lot of gia ex cuts to the curb depending on whats in the averages.
There is no way to tell where that combo falls by the numbers because of the gia rounding.
 

Karl_K

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The diamond is not what's considered as a balanced stone, but it can be a very fiery (lots of colored light vs white light) stone. Without a photo/video or ASET/Idescope image, there's no telling how this diamond would perform.
While its a different balance point than near tolk h&a cuts if it falls in the right actual angle range with in the rounding, in no way is it unbalanced as in bad.
Just different.
 

Bron357

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I am fascinated by these threads, who knew (outside Pricescope) that diamond evaluation was so intricate and technical.
I think 99% of people who bought a diamond just went.....
“Oh, that’s so pretty, how much?”
 

yssie

Super_Ideal_Rock
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@yssie, when running GIA facetware and entering this data why does it kick back a pavilion & overall depth that disagree with the cert? In this case it says pavilion is 42.6 (vs 42.5) and overall is 62.6 (vs 62.3).

Just wondering if we are teter-totering between VG and EX, if facetware kicking back values that are slightly different would force it one way or the other?
Screenshot_20190118-132232_GIA Facetware.jpg

Definitely agree this is the takehome!

I wasn't sure how Facetware calculates those values (crown height, pav depth, total depth). The help does explain that these values may differ from what's printed on the report for a given stone:
https://www.gia.edu/facetware-help

The total depth percentage displayed in Facetware® may differ from the value printed on the report. For GIA Diamond Grading and Diamond Dossier® Reports, total depth percentage is based on the measured diameter and depth of the diamond. The total depth percentage displayed in the Facetware® output is calculated from the combination of rounded proportion parameters, according to this formula: Crown Height % + Pavilion Depth % + Girdle Thickness %, where crown height and pavilion depth percentages are derived from the calculations mentioned above. This calculated value may differ from the reported total depth percentage; typical differences are 0.2 – 0.3%, with extreme cases differing by as much as 0.8%.
The crown height percentage displayed in Facetware® may differ from the value printed on the report. For GIA Diamond Grading and Diamond Dossier® Reports, crown height is measured at the eight bezel facets. These eight measurements are then averaged and converted to a percentage of the average diameter. The resulting number is rounded to the nearest 0.5%. Facetware® calculates crown height according to this formula, using roundedtable percentage and rounded crown angle: Crown Height % = 0.5 (100 - Table %) x TAN (Crown Angle Average). The resulting number is rounded to one decimal place.
The pavilion depth percentage displayed in Facetware® may differ from the value printed on the report. For GIA Diamond Grading and Diamond Dossier® Reports, pavilion depth is measured at the eight pavilion main facets. These eight measurements are averaged, converted to a percentage of the average diameter, and rounded to the nearest 0.5%. Facetware® uses culet size and rounded pavilion angle to calculate pavilion depth according to this formula: Pavilion Depth % = 0.5 (100 – Culet Size %) x TAN (Pavilion Angle Average). The resulting number is rounded to one decimal place. A specific value is used in this formula for each verbal culet size; very small corresponds to 0.5%.


I know the GIA (lab) measures total depth, and calculates crown height percent and pavilion depth percent based on measured crown height and pavilion depth:

[measured vertical distance from table plane to point where kite facet meets girdle, averaged around the stone] / [girdle average diameter] * 100

[measured vertical distance from culet plane/tip to point where main meets girdle, averaged around the stone] / [girdle average diameter] * 100

So I think it is safe to say that the differential is caused by measurement (subject to scanner error and averaging) vs. calculation (subject to aggregate error from function inputs). So I guess you get to define your own "truth" :mrgreen2:
 
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sledge

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Thank you @yssie and @gm89uk. Much appreciated.

So short answer -- rounding & averaging errors.

In laymen's terms, we use rounded & averaged data from the GIA report and input into Facetware. Some additional assumptions and math is performed. Some additional minor rounding is added for good measure. And then we end up with variances like I pointed out.

Because so much data entered into Facetware was calculated, averaged and/or rounded, it's not unreasonable to think better input data that wasn't as manipulated would yield minor differences that may have resulted in EX instead of VG rating.

Which is essentially what @Karl_K said earlier, and what I was thinking when I asked my question. It all hinges on the actual measurements of the 8 individual crown & pavilion facet angles.
 

blueMA

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While its a different balance point than near tolk h&a cuts if it falls in the right actual angle range with in the rounding, in no way is it unbalanced as in bad.
Just different.
No one said it was "bad" @Karl_K
I suppose I should've elaborated better.
Again, stones with crown angles above 35.5, if precision cut, would be categorized as a FIC.
Subjective opinions aside, my personal preference is toward stones that tend to take on more brighter look. However, I do enjoy higher crown stones in dimmer lighting.
Read more here
bic.gif
fic.gif

http://www.diamond-cut.com.au/23_bicfic.htm
 
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sledge

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If table is exactly 56%, with exact 36degree crown, then CH is 15.98% or 16% (tan36)(1-0.56)/2
If culet is pointed pavilion angle is exactly 40.6, then PH is 42.86% (tan40.6/2)
If the girdle is exactly 4% then in total it's 4 +15.98 + 42.86 = 62.84% depth.

For those desiring to use Excel to do this work, you have to remember that Excel uses radians as it's default input method. The above information is reported in angles.

This can be resolved by using =RADIANS(36) for instance and it will convert 36 degrees to 0.628319 radians.

More simply, you can nest formulas such as these:
  • Crown Height =TAN(RADIANS(36))*(1-0.56)/2
  • Pavilion Height =TAN(RADIANS(40.6))/2
  • Items in red are specific to the diamond you are looking at.
    • For crown height the 36 is the GIA reported crown angle, and 0.56 is the GIA reported table value of 56% (or 56/100).
    • For pavilion height the 40.6 is the GIA reported pavilion angle.
 

Matilda

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John P

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No need to overthink why GIA dinged it to VG.
  • The numbers on the report (after rounding) are GIA gospel
  • The main proportions get EX
  • Unless girdle is too shallow or thick, which is reflected in depth %
  • Indeed, girdle went across GIA limit for these main proportions
  • At that point even these main proportions get dinged to VG
Girdle/depth: @skypie and @yssie for the win.

But girdle thickness won't impact its optics. At face value it could be a stunner.
 

sledge

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If you find yourself needing to convert crown or pavilion heights to angles you can use these formulas that @asblackrock provided in the original thread below.

My work is limited to the Excel formulas only.

https://www.pricescope.com/communit...-calculating-crown-and-pavilion-angles.20396/

PAVILION ANGLE:
1. Enter the Pavilion depth into your calculator.
2. Divide this by 50.
3. There is a button on many calculators labelled “Tan –1”, or it may be written in small writing above the “Tan” button. Press the “Tan –1” button (or if it is written above the “Tan” button, press the shift button, then the “Tan” button). This is the exact pavilion angle in degrees.

Eg. Pavilion depth = 43.1
43.1/50 = 0.862
“Tan –1” 0.862 = 40.76. This is the exact pavilion angle in degrees

The tan-1 function is equivalent to Excel's ATAN function. Also, because of the radians thing I noted earlier, you want to output your answer to degrees so we will use the DEGREE function as well. All nested together it looks like this:

  • Excel version: =DEGREES(ATAN(43.1/50))
    • where 43.1 is actual pavilion height shown on report

Keep in mind, because the way GIA rounds/approximates/calculates pavilion & crown heights you sometimes get funky results vs if you try this. Point in case is if you input this particular stone's pavilion height of 42.5. Reality is 40.6 is closer to 42.8% but GIA tends to round that value down to 42.5, which gives a bunk result in Excel.


CROWN ANGLE:
1. Divide the Table by 100
2.Subtract this from 1
3. Multiply this by 50
4. Store this figure
5. Divide the Crown depth by the above figure
6. Press the “Tan-1” key. This is the exact crown angle in degrees

Eg. Table = 53 Crown depth=16.2
53/100 = 0.53
1-0.53 = 0.47
50x0.47 = 23.5
16.2/23.5 = 0.68936
“Tan –1” = 34.58. This is the exact crown angle in degrees

  • Excel version: =DEGREES(ATAN(16.2/((1-(53/100))*50)))
    • where 16.2 is reported crown height
    • where 53 is reported table
 

blueMA

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For those desiring to use Excel to do this work, you have to remember that Excel uses radians as it's default input method. The above information is reported in angles.

This can be resolved by using =RADIANS(36) for instance and it will convert 36 degrees to 0.628319 radians.

More simply, you can nest formulas such as these:
  • Crown Height =TAN(RADIANS(36))*(1-0.56)/2
  • Pavilion Height =TAN(RADIANS(40.6))/2
  • Items in red are specific to the diamond you are looking at.
    • For crown height the 36 is the GIA reported crown angle, and 0.56 is the GIA reported table value of 56% (or 56/100).
    • For pavilion height the 40.6 is the GIA reported pavilion angle.

@sledge you're such a geek, but I mean that in an endearing way. :P2
We're all tired of this silly GIA rounding game. I can't wait for them to start changing the reporting structure. They changed majorly once already, but they're so slow to change and I still have a few diamonds with their older reports before they started listing the crown/pav angles out of the AGS pressure. So annoying...
 

gm89uk

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No need to overthink why GIA dinged it to VG.
  • The numbers on the report (after rounding) are GIA gospel
  • The main proportions get EX
  • Unless girdle is too shallow or thick, which is reflected in depth %
  • Indeed, girdle went across GIA limit for these main proportions
  • At that point even these main proportions get dinged to VG
Girdle/depth: @skypie and @yssie for the win.

But girdle thickness won't impact its optics. At face value it could be a stunner.

But this diamond is medium to slightly thick girdle at an average of 4% which is a well accepted tolerance to my knowledge for GIA Excellent. I did not know GIA had different tolerances for girdle for different proportions.

Ultimately there are lots of other reasons which are not evident on the report, that it could have been dinged to VG.
 

kmoro

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John P

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But this diamond is medium to slightly thick girdle at an average of 4% which is a well accepted tolerance to my knowledge for GIA Excellent. I did not know GIA had different tolerances for girdle for different proportions.

Ultimately there are lots of other reasons which are not evident on the report, that it could have been dinged to VG.
Not this one, though @gm89uk . I can be more clear. This is not about a 4% girdle. It's the consideration of everything together. It's why I first referenced "the main proportions" above. Those main proportions are 56T 36C 40.6P. Adding a girdle of 4% created too much depth for this combo, thus VG. If the crown were shallower (35.5 even) a 4% girdle would not be an issue.

The cutting motivation is clear. Thicken the girdle and save 0.80 carats. The producers knew full well the ding to VG was coming and swallowed it. Why? Because in most jewelry stores a high color E-VS2 can be sold more quickly as 0.80 VG than it can be as an equally pretty 0.79 EX.
 
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