Diffs between lab, optical and physical symmetry and effect on cut?

[Lula]

I've been here only a little while (@six months) and have learned a lot. Thank you, everyone!

However, there are still several terms that I do not fully understand, and I'm wondering if the experts would help me out by defining lab, optical, and physical symmetry, and explain the differences -- in layperson's terms?

For example, what differences would I be able to see, if any, between 2 stones, one having poor optical symmetry and one having excellent optical symmetry? Does having excellent optical symmetry guarantee excellent physical symmetry or vice versa?

Also, what difference would it make for these two stones' grading under the current AGS/GIA systems?

(I know, I don't ask for much, do I?)

But these terms get thrown around from time to time on threads, and they can be really confusing. I've seen Storm categorize them this way

meet point/lab == symmetry
optical == alignment
physical/scanned == tightness

which is helpful, but I could use a more fleshed out, shall we say, definition of each!

Thanks...

 

[stone-cold11]

Read this article before?

http://www.gemology.ru/cut/english/symmetry/

 

[Lula]

No, I haven''t seen that article, Stone -- it looks good, thanks!

 

[Garry H (Cut Nut)]

If we are just talking round diamonds - then maybe size has an important role as well.
e.g. sym is maybe not so important in 0.10ct and some types of lower sym can be an advantage in +5ct.
Around 1ct top sym seems to be an advantage.

 

[Rhino]

Hey Sara,

Off the cuff ...

Date: 9/16/2009 1:45:53 PM
Author:sarap333
I''ve been here only a little while (@six months) and have learned a lot. Thank you, everyone!

However, there are still several terms that I do not fully understand, and I''m wondering if the experts would help me out by defining lab, optical, and physical symmetry, and explain the differences -- in layperson''s terms?

Well ... here''s the Rhinocerotic "nutshell" version. Lab graded symmetry/ie. physical symmetry is an analysis of how well and aligned all facet points meet as the exterior of the diamond is analyzed. This is also referred to as meet-point symmetry but there are other features that are examined in the lab such as off centered table, off centered culet, wavy girdle, etc. We have the entire chart of these in our tutorial as well as a high definition video demonstrating these features under a microscope.

Optical symmetry on the other hand is an examination of the optical alignment of the reflections observed within the diamond particularly through optical devices that demonstrate this level of craftsmanship. It''s akin to clarity grading through a microscope except we''re applying the same principals to cut grading through an H&A viewer.

This alignment can and does result in visual differences and can even impact cut grades for light performance (particularly within AGS''s sytem) even though AGS doesn''t necessarily grade for Optical Symmetry. Not too long ago we had 2 GIA Ex''s with identical proportions. One had PGS score of "0" and the other a "3". The primary difference between the 2 was Optical Symmetry.

Optical Symmetry is also a feature that can be confirmed mathematically by an examination of slope and azimuth angles of a diamond.

For example, what differences would I be able to see, if any, between 2 stones, one having poor optical symmetry and one having excellent optical symmetry?

Depends on the comparison but a diamond with what I would grade as "common" or "traditional" optical symmetry ... I can visually see the differences in optical alignment. The diamond with superior optical symmetry will look symmetrical when observed particularly in diffused type lighting which empahsizes brightness and contrast. A diamond with traditional optical symmetry will look asymmetrical or chaotic in that same lighting. I have an example of this in my article on the subject.

Does having excellent optical symmetry guarantee excellent physical symmetry or vice versa?

Nope. You can have superior Optical Symmetry yet not have Ideal or Excellent symmetry & vice versa. Excellent lab/physical symmetry does not = superior Optical Symmetry.

Also, what difference would it make for these two stones'' grading under the current AGS/GIA systems?

In GIA''s system it would make no difference since there is a certain degree of rounding involved with no consideration to optical symmetry. When you get down to it, it is a numbers based system plus a conservative checking for painting and digging (girdle cutting features).

In AGS'' system, while they do not consider optical symmetry, it is a fact that optical symmetry can improve the light return of a diamond. To date the most I have seen it affect a light return grade was 3 grades.

(I know, I don''t ask for much, do I?)

But these terms get thrown around from time to time on threads, and they can be really confusing. I''ve seen Storm categorize them this way

meet point/lab == symmetry
optical == alignment
physical/scanned == tightness

which is helpful, but I could use a more fleshed out, shall we say, definition of each!

Thanks...

Oh yea ... physical symmetry.

You can only see that effecticvely on each facet set through a Helium Report. Some Sarin''s online show min/max crown angles and/or pavilion angles but those are only 2 facet sets. Hope that helps.

Kind regards,

 

[Lorelei]

Date: 9/17/2009 12:50:31 AM
Author: Rhino
Hey Sara,

Off the cuff ...

Date: 9/16/2009 1:45:53 PM
Author:sarap333
I''ve been here only a little while (@six months) and have learned a lot. Thank you, everyone!

However, there are still several terms that I do not fully understand, and I''m wondering if the experts would help me out by defining lab, optical, and physical symmetry, and explain the differences -- in layperson''s terms?

Well ... here''s the Rhinocerotic ''nutshell'' version. Lab graded symmetry/ie. physical symmetry is an analysis of how well and aligned all facet points meet as the exterior of the diamond is analyzed. This is also referred to as meet-point symmetry but there are other features that are examined in the lab such as off centered table, off centered culet, wavy girdle, etc. We have the entire chart of these in our tutorial as well as a high definition video demonstrating these features under a microscope.

Optical symmetry on the other hand is an examination of the optical alignment of the reflections observed within the diamond particularly through optical devices that demonstrate this level of craftsmanship. It''s akin to clarity grading through a microscope except we''re applying the same principals to cut grading through an H&A viewer.

This alignment can and does result in visual differences and can even impact cut grades for light performance (particularly within AGS''s sytem) even though AGS doesn''t necessarily grade for Optical Symmetry. Not too long ago we had 2 GIA Ex''s with identical proportions. One had PGS score of ''0'' and the other a ''3''. The primary difference between the 2 was Optical Symmetry.

Optical Symmetry is also a feature that can be confirmed mathematically by an examination of slope and azimuth angles of a diamond.

For example, what differences would I be able to see, if any, between 2 stones, one having poor optical symmetry and one having excellent optical symmetry?

Depends on the comparison but a diamond with what I would grade as ''common'' or ''traditional'' optical symmetry ... I can visually see the differences in optical alignment. The diamond with superior optical symmetry will look symmetrical when observed particularly in diffused type lighting which empahsizes brightness and contrast. A diamond with traditional optical symmetry will look asymmetrical or chaotic in that same lighting. I have an example of this in my article on the subject.

Does having excellent optical symmetry guarantee excellent physical symmetry or vice versa?

Nope. You can have superior Optical Symmetry yet not have Ideal or Excellent symmetry & vice versa. Excellent lab/physical symmetry does not = superior Optical Symmetry.

Also, what difference would it make for these two stones'' grading under the current AGS/GIA systems?

In GIA''s system it would make no difference since there is a certain degree of rounding involved with no consideration to optical symmetry. When you get down to it, it is a numbers based system plus a conservative checking for painting and digging (girdle cutting features).

In AGS'' system, while they do not consider optical symmetry, it is a fact that optical symmetry can improve the light return of a diamond. To date the most I have seen it affect a light return grade was 3 grades.

(I know, I don''t ask for much, do I?)

But these terms get thrown around from time to time on threads, and they can be really confusing. I''ve seen Storm categorize them this way

meet point/lab == symmetry
optical == alignment
physical/scanned == tightness

which is helpful, but I could use a more fleshed out, shall we say, definition of each!

Thanks...

Oh yea ... physical symmetry.

You can only see that effecticvely on each facet set through a Helium Report. Some Sarin''s online show min/max crown angles and/or pavilion angles but those are only 2 facet sets. Hope that helps.

Kind regards,

Great post Jon!!

 

[Lula]

Thank you so much, Rhino, for a great explanation. I need some more time to digest this -- I know I''ll have a few more questions

 

[Rhino]

No sweat. I''ll subscribe here and answer as time allows Sara. If there is anything that''s not clear I''ll help expound.

Kind regards,