Calculating girdle thickness

[harvey]

Quick question, I''m looking at a round diamond g color, Si2(eyeclean), 58% table, 62% depth, 44%pavillion depth, 14.7% crown height, medium girdle, symmetry/polish VG, no flourscence, 1.22 carat, 6.9-6.87 * 4.27, meduium girdle. It is an EGL-USA cert(12/03) (I think LA). Question #1: Am I correct in assuming that since the pavillion and crown heights add up to 58.7% that the girdle constitutes 3.3% (2.27mm). Working on this presumption, with some simple trig, I calculated the crown angle to be 31.3, and pavillion angle to be 41.29. The angles give a better reading than the percentages using the HCA. The percentages actually give a poor score. The diamond looks amazing. I have now looked at >50 diamonds and this one is up there with the best I have seen in my price range. It is listed as an "ideal plus" on the cert. I know it is not ideal just by looking at the table and pavillion depth. Am I putting too much credence into the HCA using percentages alone. From what I have read the HCA score usually worsens with angle measurements. Why not in this case? Is it b/c HCA assumes girdle of 1.7%? Any thoughts.

 

[oldminer]

You are no doubt an engineer or some other math whiz. The numbers on diamonds do not add up like one might expect, so your calculations of girdle thickness are likely incorrect. The other calculations of angles may be technically right, but probably they are off a bit, too. The devices which measure diamonds have some inherent error that makes no difference to diamond guys, but makes engineer folks wild about the lack of accuracy. Just be very thankful that diamond sellers and gemologists don't build bridges.

Give your eyes some credit for being able to know a nice diamond versus a poor one. It takes a lot of looking to discern fine stones from very good ones.

The weak link in your chain of "facts" is the EGL-LA cert rather than a GIA or AGS paper. Every dealer discounts the accuracy of those documents and you probably would be well advised to also downgrade the color and possibly the clarity on the report. It won't change the diamond, but it will change the dollars that one might estimate for its value.

 

[harvey]

oldminer, do you have bad experience with the EGL-usa certs. From searching on the forum people think that they are not unreliable like their true european counterparts. I actaully called Egl-la and they gave me the crown angle of 35 degrees, which is off from my calculation. So like you said I guess the measurements just don't add up like we think they do. They did not have the pavillion angle. Is 44% too deep? It seems like this is the one # that may be off in this diamond. Is there any credence to be put in the "ideal plus" cut classification? The diamond is brighter and more fiery than others that I looked at with higher scores on the HCA. Also price is 4500$ with setting included (4-prong solitare)from a very reputable local jeweler. Any opinions on this price.

 

[oldminer]

I am not truly an active diamond dealer. Most of my expertise is in the appraisal arena, although I certainly specialize in old European cut diamonds, but only sell them to members of the jewelry trade. I respect the new management of EGL-USA which includes the LA lab. Are they the GIA? NO.
Do EGL graded diamonds bring the same price as GIA graded diamonds based on the color and clarity EGL calls? NO.

It is simple enough to judge that the trade believes that the GIA grades "tougher". The fact is that a true H color SI1 diamond trades for about the same price regardless of which lab certed it. One lab might call it G/VS2 and another might call it I SI1, but if it truly is H/SI1, it will command that price from knowledgeable dealers who buy the diamond, not the paper.....

Take a look at the cut class charts on www.gemappraisers.com and see what my opinion of the cut might be. It is a way to gain some further insight, but your eyes remain the best and final judge.

 

[harvey]

Cut: Round
Weight: 1.22 carats
Table % 58 Grade: 1A
Crown Angle 35 Grade: 1B
Crown Height % 14.7 Grade: 1A
Pavilion Depth % 44 Grade: 2A
Girdle Thickness Medium to Medium Grade: 1A
Total Depth % 62 Grade: 1A
Polish / Symmetry Excellent Grade: 1A * Not a primary determining factor.

Final Grade: 1B

In addition when I apply the AGS criteria the diamond scores the following AGS grades(0-10): crown angle-0, table-1, pav depth-1, girdle-0, cutlet-0, polish/symmetry-0.

The diamond grades well with both of the above grading schemes. I know its "all about the crown and pavillion angles" the latter of which I do not have a direct reading on, but do these cut grades mean nothing with a poor HCA score or do the good cut grades with the amazing look of the diamond mean that the HCA is not always the "best" way to analyze a diamonds performance?