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Tiffany vs. GIA -- discrepancy on pavilion depth

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lainghk

Rough_Rock
Joined
Apr 19, 2006
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I purchased my engagement ring from Tiffany a few years back and the diamond came with both Tiffany cert. and GIA cert (old format without cut grade). The cut grade is "excellent" on the Tiffany cert. Recently I obtained an updated GIA cert (new format with cut grade) for the same diamond and the GIA cut grade is "very good" on the GIA cert. I noticed there is one discrepancy on the Tiffany vs. GIA cert in terms of the "pavilion depth": 42.8% on Tiffany's vs. 42.5% on GIA's (GIA cert also shows pavilion angle of 40.6 degree but that figure is not available on the Tiffany cert).

Questions:

1) Is there a formula for calculating pavilion depth based on pavilion angle?
2) How significant is the 0.3 difference on the pavilion depth?
3) Who likely made an error on the pavilion depth (Tiffany or GIA)?
4) Should I approach Tiffany for a re-cert (I believe Tiffany's new cert. format now includes pavilion angle as well)? What are the chances that Tiffany would entertain such a request?

Any suggestion would be greatly appreciated!

P.S. I put the #'s through HCA and all three combinations produced favorable TIC results:

Depth %: 61.1
Table %: 55
Crown angle: 35
1) Pavilion depth: 42.8 (Tiffany)
2) Pavilion depth: 42.5 (GIA)
3) Pavilion angle: 40.6 (GIA)
Culet %: 0 (NONE)

Resulting TIC:
1) 0.6 (Tiffany)
2) 0.6 (GIA)
3) 0.8 (GIA)
 
GIA rounds their numbers for crown and pavillion angles, so that''s probably what accounts for the discrepancy.
 
Hi there, and welcome to Pricescope!

To try to answer your questions:

1. Yes, assuming basic trigonometry, the formula is 0.5*tan(Pavilion angle). I calculated it using 40.6 and came up with a depth of 42.8%.

2. That difference is almost nothing. You ran it through the HCA and it looks good for all combos, so I wouldn''t worry about it.

3. Neither made an error. GIA rounds most of their numbers. I believe they round to the nearest 0.5% on depth and table but to the nearest 0.1 deg on pavilion angle while only the nearest 0.2 deg on crown angle (I think). My guess is that neither made a mistake, one is just rounded.

4. No, I wouldn''t bother. The cert isn''t going to change the diamond, its just a piece of paper. If you''re happy with the diamond, and it sounds like you are, it doesn''t matter what the pavilion angle or depth is. That change won''t affect the appraisal value, which is the only reason you would get a new cert.

My advice is to just be happy with your ring! It sounds like a wonderful diamond and those are great HCA scores, no matter which combo you go with.
 
Thank you very much dockman3! I am very happy with my diamond. It''s just this one thing that has been bothering me since I received the new GIA report with the cut grade: while Tiffany gives "Excellent" on its precision of cut, GIA gives it "Very Good" (I would have expected "Excellent" on GIA as well).

Researching a little further and inputting the parameters from the new GIA cert. into GIA''s Facetware Cut Estimator and comparing with GIA''s Facetware Look-up Tables (http://www.diamondcut.gia.edu/08_tools_for_the_trade.html), I found that one single parameter could have changed the cut grade from "Very Good" to "Excellent" by GIA''s standard -- it''s the "Lower Half %": 85% ("Very Good" cut grade as on my new GIA cert) vs. 80% ("Excellent" cut grade as on GIA Facetware) given the same angle/pavilion specifications.

I am not sure if Tiffany''s new grading report uses "Lower Half %" and "Star Length %" terminologies like GIA, even if I have my diamond re-graded by Tiffany? Could GIA have made an rounding error on the "Lower Half %" calculation (5% is the minimum increment/decrement)? Does AGS also use "Lower Half %" and "Star Length %" in its grading system?
 
I believe they both (GIA and AGS) use lower half % and star length, and yes, GIA probably made a huge rounding error when they round to the nearest 5%. That''s a huge amount to round, and it sounds like you are right on the border. I don''t know anything about Tiffany''s grading system, but I"m sure they take all of these things into account as well.

I, personally, don''t think that a regrade by Tiffany is going to do anything. They graded it once, and unless their grading criteria have changed, they will probably regrade it the same. Try not to be concerned with what the new GIA report says. There are many people on this board who will tell you that a GIA VG cut can outperform a GIA Ex cut diamond. Its not unheard of and though I understand your concern, this is just one of those times. The GIA cut grade is not fool-proof and this is just one of those strange things that doesn''t really effect the overall performance much, but in their number scheme, for some reason it grades as VG. It is still an absolutely beautiful diamond, and as long as you love it, who cares?!
 
Thanks again dockman3. My wife would love your answers very much!
 
Glad I could help!
 
Adding to dockman’s astute observations, GIA doesn’t round depth or table: They report to the nearest 0.1% and 1% respectively. They round pavilion angle to nearest 0.2 degree, crown angle to nearest 0.5 degree and stars and lower halves to nearest 5%.
 
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