Colorimeter questions...

[MichaelF]

I just got back from my appraisal and I have a question about the grading of color using a "Colorimeter" and "Visual Comparison" against CZ''s matching grading standards... Here''s my dilema. The AGS reports grades the stone as a "G", Visual Comparison grades it "H" and the Colorimeter comes in at a "K" or "L"... My questions follow:

1. What method / report should be considered most accurate considering I''m using a highly recommended appraiser...

2. Can a Colorimeter be "out of calibration"?

3. Should I go back to my merchant and ask for $ consideration for the color difference?

I love the stone and this is my only issue / concern...

Michael

 

[MichaelF]

Oh... by the way... How does a Colorimeter work?

 

[smaggard]

K or L is quite different from G.

Maybe the colorimeter needs calibration? It's my uderstanding, as explained by Rockdoc, that the Gran colorimeter is very accurate for colors I and lower, but it sometimes grades H and higher stones a half grade better than they are.

Also I know some appraisers, like RockDoc use GIA master diamonds, not CZ's. Not sure if this could make a difference, but those are big discrepancies it would seem.

Personally I would feel better about spending lots of money on something that came back as it was advertised. But if you like it and they could knock off some $$$, you could get a good buy perhaps.

The experts are sure to know more though

Good luck!

 

[skibum]

Houston....we have a problem.




Cheers,
David

 

[skibum]

This is a good question for all you diamond techies out there. If a good lab (GIA or AGS) says a stone is "G" and a colorimeter says it's "K", who do you believe? Is it worth re-appraising the stone? What if it come back a "K" again? Would a dealer send the stone back to the lab to verify? What does that do to the price.

Cheers,
David

 

[niceice]

With all due respect to RockDoc, we don't use a Colorimeter because we played with them and discovered that they were easily fooled by simple things like a thumbprint. A lab graded set of "master stones" is much more accurate with a little practice and the right lighting conditions. The AGSL uses both a master set of diamond grading stones for grading color AND an extremely precise machine and a specialized version of Marty Haske's SAS2000 which is kind of like a colorimeter on steroids, we're inclined to think that the grade assigned to the diamond by the lab is more accurate than the one assigned by the appraiser but you can always send the diamond back to the AGSL for verification... Does the diamond look kind of yellowish-brown to you from a side profile? Kind of like the color of a dog's urine on a sliding glass door? If not, it's probably not a K color

Oh yea, those CZ Master Sets aren't all that accurate to begin with and they fade / change color after about a year and should be replaced as frequently according to most manufacturers so we wouldn't place to much weight in those critters...

 

[MichaelF]

NiceIce,

Thanks for the reply... I'm incline to go with the lab on this one too... No, there seemed to be almost no, none, zilch color on the pavilion from the side. I had a very difficult time seeing this stone as a "G" even before the testing. I saw an "I" at a B&M and there was a definite pale yellow on the pavilion and this stone has not a trace of yellow to my untrained unprofessional eye. The H&A's are awesome, idealscope seems outstanding, it scores a nice 1.3 on the HCA (details in a prior post)...

Michael

 

[Richard Sherwood]

A colorimeter doesn't have the consistency or accuracy of a human grader. So it comes down to the appraiser's "H" versus the AGS "G". As NiceIce pointed out, the AGS has the master diamonds and instrumentation in it's favor, so without knowing the appraiser, I would tend to lean towards AGS's opinion.

 

[Garry H (Cut Nut)]

I have been through the AGSL and they actually use a cheap colorimeter as part of the checking process (but never to be relied upon).

Your stone probably has a peculiar fluoresence and has fooled the machine.

The AGS grading will be correct.

Does anyone know if AGS have also dropped the use of UV filters as have GIA? That could easily explain the difference.

 

[MichaelF]

Cut Nut...

I agree with you regarding the Colorimeter reading... As in any sample wide variations should probably be discarded and this is between G and H not G and K/L... Regarding florescence, a test was done and the AGS rated the stone as having negligible florescence, the appraiser found none...

If it were a K / L I believe the pavillion would have been fairly yellow to the eye, and there was no indication of that... If the CZ's fade, as someone else pointed out, could the appraiser's "H" CZ be more akin to a "G" diamond? Or,is this just reaching too far? On the other hand, this is a well respected merchant and appraiser with years of experience. Wouldn't they know that CZ's fade? I'm a bit confused...


Michael

 

[Garry H (Cut Nut)]

Clever people keep CZ away from UV lights and out of daylight then they are stable for decades.

If you are in doubt get Dave Atlas to adjudicate

 

[MichaelF]

Cut Nut,

I am assuming from your reply that UV light from natural or artifical sources is what causes the CZ standards to fade. The UV light used to test for florescence was, part of the color grading setup. The CZ standards were on a row / bench to the rear of a white box and the light was switched from bright white (daylight) to black (UV) and shined down on both the diamond and the CZ. While this was only for a few moments, I suppose it could have ben a few moments a few times a day, five days a week, four weeks a month... Plus the display / test was close to an exterior window. Possible source of sunlight? Real me back in if I'm too far in right field, but perhaps the issue is with the CZ's having faded so that their CZ "H" looks like the "G" diamond?

This would explain a lot, making everyone technically right. The AGS report is right as it says "G", and the appraiser compares the stone to the CZ most closely matching the diamond and voila... It's an "H", but only because of the condition of the CZ...

Michael...

 

[Garry H (Cut Nut)]

We use both diamond sets and CZ and we keep our UV is in a different room.

Sounnds like you have your answer

Carless behaviour.