Opinion on this GIA 0.74 I found

[shinji1]

Hi all, I recently found a H&A 0.74 diamond from a local jeweler, would like your opinion on it. Here''s the link to the GIA report :

http://www.gia.edu/reportcheck/index.cfm?fuseaction=home.showReportVerification&reportno=14206686&weight=0.74

I viewed this diamond under a "ideal scope" and observed the H&A pattern. Its nice.. but the "cut grade" on the gia report is only very good. Is that cause for concern?

The seller also told me that a D colored diamond will give off more fire and brilliance then a similar diamond with a lower color rating. He also suggested that a diamond with no fluorescence is the most desirable.

Thanks for any help in advance.

 

[JulieN]

VG cut grade due to shallow/shallow combo. I''d pass.

A D could have the potential to return more light, but it''s all in the cut. IF you wanted to optimize a D and you wanted to optimize a G, they would be cut to different proportions.

 

[shinji1]

Date: 4/16/2007 5:11:34 PM
Author: JulieN
VG cut grade due to shallow/shallow combo. I'd pass.


A D could have the potential to return more light, but it's all in the cut. IF you wanted to optimize a D and you wanted to optimize a G, they would be cut to different proportions.

Can you please explain what you mean by Shallow/shallow combo? Sorry I'm pretty new at this.

Thanks in advance!

I popped the parameters into the HCA, and I got a rating of 0.7, which supposedly is within the TIC range. What I'm not sure is what that number (0.7) means. Is small better?

 

[JulieN]

Here are the Tolkowsky proportions: http://diamonds.pricescope.com/tolk.asp

The .74:
Depth: 60.9 %

Table: 56 %

Crown Angle: 33.5°

Crown Height: 14.5 %

Pavilion Angle: 40.6°

Pavilion Depth: 42.5 %

So the crown angle is shallower than Tolk (by a degree-yes, that''s a lot) and the pavilion angle as well is shallower than Tolk. Then we call it a shallow/shallow combination.

 

[JulieN]

For HCA, anything under 2 can be rejected, and lower is not better. There are many combos that are under 2. What HCA does is gives us a wider range of ideal, rather than just the Tolkowsky standard.

 

[JohnQuixote]

Hey Julie, at the risk of picking nits

I'd refer to this simply as 'shallow' using GIA nomenclature. Shallow-shallow would see the CA nearer 31.5 degrees.

GIA Definitions

Slightly shallow crown = at or near 31.5
Moderately shallow crown = at or near 26.5
Shallow crown = at or near 22.0
Very shallow crown = at or near 20.0
Extremely shallow crown < 20.0

Slightly shallow pavilion = at or near 40.4
Moderately shallow pavilion = at or near 39.8
Shallow pavilion = at or near 38.8
Very shallow pavilion = at or near 37.4
Extremely shallow pavilion < 37.4

 

[shinji1]

Date: 4/16/2007 5:21:18 PM
Author: JulieN
For HCA, anything under 2 can be rejected, and lower is not better. There are many combos that are under 2. What HCA does is gives us a wider range of ideal, rather than just the Tolkowsky standard.

Thanks for the information. Some more questions about the HCA :
1. Is it true that the HCA is only meaningful for diamonds that are perfectly symmetric? On the same token, since H&A diamonds are perfectly symmetric, would that imply that HCA is only meaningful for H&A diamonds?
2. I''ve seen many AGS0 having a HCA rating lower than 2. I know this might not be a valid question, but what does this mean?
3. "Diamonds that rate below 2 (red on the chart) are unlikely to show too much leakage darkness, overly thin or thick girdles, or fish-eyes". Isn''t this something we want? If so, why is a diamond with a HCA < 2 undesirable?

Thanks again for your help

 

[JulieN]

Ah, yes, John.

shinji,
1. HCA gives the most accurate numbers when the stone is cut most accurately. Makes sense, doesn't it? On HCA, we are just aiming for under 2. 1.2 is not better than 1.5, so we are just asking for a reasonable amount of symmetry.
2. That means AGS's scoring and the HCA theory is aligned. Or something like that.
3. MY BAD! Stones with HCA greater than 2 are to be rejected.

 

[SBee]

Date: 4/16/2007 5:21:18 PM
Author: JulieN
For HCA, anything under 2 can be rejected...

is that a typo?? I'm confused. I thought it was the other way around.
*EDIT* NEVERMIND! We posted at the same time

 

[shinji1]

Thanks for all the replies! You guys are so helpful.

One thing I''m still not certain about is fluorescence, and how will it affect the brilliance/fire of the diamond. Is it true that we should strive for rocks that have no fluorescence?

Also how important is it for a diamond to have a color rating of D, compared to say G? Will the color of the diamond affect the perceived fire/brilliance of the rock?

 

[JulieN]

What is fluorescence? Light in the UV range is brought down to a lower energy level and emitted in the visible range. That doesn''t sound like a reason to avoid it, does it?

Many people cannot tell the difference between D and G face up, anyway. As long as they are both properly cut, there will not be any visible difference in performance.