closing in on a purchase, any last advice (ags 0 questions)

[johngalt2004]

After much drama I think I will be going in today or maybe tomorrow to look at 3-4 ags0 stones. I don''t expect to have an idealscope handy so I would like some advice. I know the key measures of two of the stones:

a)
1.17, H, SI1, $6600
55 tbl
60.4 depth
34.8 CA
40.9 PA
1.6 HCA

b)
1.17, H, SI2, $6200
57 tbl
61.3 depth
34.6 CA
41 PA (kiss of death?)
2.0 HCA

Among the other choices will be an SI2 ags 0 with specs I don''t know yet - about the same size for $4400! Gotta think that one has a grisly inclusion for that price but I guess we''re going to take a look.

So my questions are these:

Are either of the stones above likely to be "bad zeros"? Are either likely to be stellar?

How firm is the 2.0 HCA threshold. As you know if the PA is 40.9 it will come up great. If 41.1 it will come up bad. So how fine can the 2.0 line really be since a calibration error can thrust the stone so far one way or the other when you are at the 41 PA mark?

Can light leakage under the table be evaluated by a gemologist credibly without an idealscope?

Is 41 Pavilion Angle a kiss of death to be avoided?

What are the pitfalls to watch out for in looking at the unknown spec stone, absent an idealscope or immediate access to HCA?

Is a light based zero, a great stone no matter what? Only if PA is <41?

Does light based zero trump HCA?

Does light based zero trump IdealScope?

 

[Regular Guy]

John,

As usual, good questions.

I''m about to go into a meeting...not that I have anything incredibly helpful to say.

To your last two...

I''d say

yes, &
probably.

Will be interested in hearing other responses.

I think not many come on here asking about bum zeros.

Regards, and good wishes!

 

[JohnQuixote]

Date: 9/20/2007 11:39:38 AM
Author:johngalt2004
After much drama I think I will be going in today or maybe tomorrow to look at 3-4 ags0 stones. I don't expect to have an idealscope handy so I would like some advice. I know the key measures of two of the stones:

a)
1.17, H, SI1, $6600
55 tbl
60.4 depth
34.8 CA
40.9 PA
1.6 HCA

b)
1.17, H, SI2, $6200
57 tbl
61.3 depth
34.6 CA
41 PA (kiss of death?)
2.0 HCA

Among the other choices will be an SI2 ags 0 with specs I don't know yet - about the same size for $4400! Gotta think that one has a grisly inclusion for that price but I guess we're going to take a look.

So my questions are these:

Are either of the stones above likely to be 'bad zeros'? Are either likely to be stellar?

Both are within pretty safe ranges. Be sure to compare them with your eyes (against less well-cut stones too) away from the jeweler's spotlights. Use diffuse (fluoro), indirect (reflected from walls, not coming from above), natural daylight and filtered natural daylight if possible.

How firm is the 2.0 HCA threshold. As you know if the PA is 40.9 it will come up great. If 41.1 it will come up bad. So how fine can the 2.0 line really be since a calibration error can thrust the stone so far one way or the other when you are at the 41 PA mark?

2.0 is a threshhold when evaluating 5 numbers but a diamond has 57 facets and some stones are ok above that threshhold. You must consider the stone as a whole.

Can light leakage under the table be evaluated by a gemologist credibly without an idealscope?

Ish. He can examine the diamond under diffused light and get an idea of dark/light ratio, brightness and pattern. For serious leakage you can place it on your hand and see your skin color through the pavilion (see this page), but that's not likely with the ones you'll be looking at.

Is 41 Pavilion Angle a kiss of death to be avoided?

It's not about one number; the combination of c/p angle is the important thing. 41/33.5 is no problem but 41/34.5 is not in such a 'sweet spot.' The reason 41 is a cutting threshhold is because the more you go above it the larger table reflection and color entrapment (below colorless grades) must be looked at - just as one should consider obstruction below 40.6. A well-cut 41.0 coupled with an appropriate CA is not going to be a problem but one where some pavilion facets stray too far above 41 will be (remember the 41 is an average of 8 numbers).

What are the pitfalls to watch out for in looking at the unknown spec stone, absent an idealscope or immediate access to HCA?

Untrained eyes may not be as cut sensitive as educated/trained ones (which can be nice too because there is no bias). Suggest you compare unknowns to known-firecrackers - and to average cuts - in many lighting conditions, as above.

Is a light based zero, a great stone no matter what? Only if PA is
See the above. With 56T the AGSL cut guides predict 0 for 41/33.4-34.2 combos...and 41.2/32.8-33.4 combos. It's possible for diamonds outside those borders to earn 0 if well-cut. They will have different looks than shallower PA/higher CA combos. LP0 is a great pedigree - it is also diverse (I'm getting deja vu here - haven't you asked this before?).

Does light based zero trump HCA?

I say yes. LP0 is a critical exam of all how all 57 facets work together. It is intended for selection. HCA is a broad prediction based on 5 numbers (2 of which are averaged) and no info on minor facets, brillianteering and cut precision. It is not intended for selection.

Does light based zero trump IdealScope?

One should verify the other.

Another question I suggest you should be asking...

Do my eyes trump everything?
Yes.

 

[aljdewey]

Date: 9/20/2007 11:39:38 AM
Author:johngalt2004

So my questions are these:

Are either of the stones above likely to be ''bad zeros''? No. Are either likely to be stellar? Yes....in fact, probably so.

How firm is the 2.0 HCA threshold. It''s not necessarily firm. It''s an arbitrary cut-off point. Not much different than saying 93-100 earns a grade of A. The student who scores a 92 is still an exceptional student....but there has to be a cut-off somewhere, and the decided upon cutoff for "A" work is 93.

Can light leakage under the table be evaluated by a gemologist credibly without an idealscope? I think it''s more realistic to expect the gemologist to offer an opinion about how fine the make is. He''s likely not going to be able to offer something ''quantifiable'' or measureable.....it will be more along the lines of "this is what I see when I look at this stone....".

Is 41 Pavilion Angle a kiss of death to be avoided? No. When paired with complementary crown angles, it''s fine.

What are the pitfalls to watch out for in looking at the unknown spec stone, absent an idealscope or immediate access to HCA? Look at it in as many light sources as possible, and have an independent source (appraiser, etc.) with no vested interest in the sale evaluate it, too.

Is a light based zero, a great stone no matter what? Pretty much, yes.

Does light based zero trump HCA? To me, yes, absolutely. Keep in mind that HCA is a "predictor" based on the rule, not the exception (and we all know there are always exceptions), and it''s done without seeing the stone. Earning a light-performance based 0 is done through evaluation of THAT actual stone.....and it''s not done making assumptions about the stone.

Does light based zero trump IdealScope? There shouldn''t be a discrepancy between the two. Both do the same thing: evaluate that stone.

 

[johngalt2004]

Thanks Alj.

John Q... thanks too, but the reason for the deja vu is that in all the searches I've done I not found a community of experts giving an authoritative nod to the concept that the light based zero has indeed taken the minute guesswork out of it all. I've been hinting for it, probing for it, leading for it.... trying to hear (only if it is true), that among several light based zeros, you can let your eyes do the picking and be very safe doing it.

Rather there have been implications that the light scoring does not really provide such comfort... some of it actually coming from Brian in that 325 post thread on the 41 Pavilion Angle (that the darkening effect is visible to eyes only, not the lab).

But I guess I am going to go on that premise... that if the guy puts 3-4 "modern zeros" in front of me, I will be able to reliably choose with my eyes from there. I guess I'm just worried about making a rookie mistake, maybe forget to look for a dark table in the shadow, or get overwhelmed by fire and choose the "poor" choice because of it. That I'll choose the unknown zero, get it home and find out it is a 2.5 on the HCA or something like that. If I just "choose with my eyes" I'll probably buy an egl and not know better. I want to buy quality in spite of my feeble eyes, impressionable psyche, and questionable taste

I'm grateful for the responses.

ps: John that is a great practical tip - I'll ask him to put a cost-no-object top of the line known firecracker out there too.... but on the other hand maybe I better not becasue then I'll just highball myself, get beyond a depth where I've done any research, and spend double my budget before I know it - buying the firecracker ... I'll think about that though.

 

[Ellen]

John, you have done everything you could do to educate yourself (to a fault perhaps?). The only thing you haven''t done is buy an IS, which I''m really beginning to think you should do, because without having some concrete validation that you''re getting a great stone, I think you may never stop questioning your choice, whatever that is.