Does good symmetry affect light performance?

[Cy2009cy]

For a cushion cut diamond, polish is ex, the symmetry is only good
Will it be a matter? How important is symmetry for a cushion? Need education.

 

[kenny]

Good proportions are the angles that result in good light return.
Symmetry is how similar all the angles are around the diamond.
Two totally different things.

There could be a diamond that is cut to horrible proportions and angles for light performance, but if all angles around the diamond are perfectly identical it could get a high symmetry grade.
IOW good symmetry can result in equally horrid proportions (angles) for light performance, all around the diamond.

You could think of cut proportions and angles that result in good light performance being the recipe.
Symmetry is how well the followed the cutter was able to recipe all around the diamond.

 

[Cy2009cy]

Good proportions are the angles that result in good light return.
Symmetry is how similar all the angles are around the diamond.

There could be a diamond that is cut to horrible proportions and angles for light performance, but if all angles around the diamond are perfectly identical it could get a high symmetry grade.
IOW it has equally horrid light proportions all around the diamond.

You could think of cut proportions and angles that result in good light performance being the recipe.
Symmetry is how well the followed the recipe all around the diamond.

Kenny, thanks for your response. So symmetry does affect light return/performance.
Is it possible a diamond with lower symmetry has great light performance as higher symmetry?

 

[shihtzulover]

I have wondered this as well, because I have been given different types of responses, depending upon who I asked. My old princess cut diamond had 'excellent' symmetry, but my new one has 'good' symmetry - I can't tell any real differences with my eyes, and I actually prefer the facet pattern of my new one.

I found this chart: https://www.pricescope.com/wiki/diamonds/diamond-grading-chart-princess/.. According to it, diamonds with 'good' symmetry can still be in the highest cut category, since it is apparently not a primary determining factor. I love my diamond regardless, but I'm still interested in hearing what other people think.

 

[kenny]

Kenny, thanks for your response. So symmetry does affect light return/performance.
Is it possible a diamond with lower symmetry has great light performance as higher symmetry?

Think of one person holding a flash light and 2 or 3 other people holding mirrors on the other side of a dark room.
If those two mirrors are at the correct angles the person holding the flash light will see their own light reflect back at them.
There are optimum angles for those two people to hold their mirrors at for max light return.

A diamond is like this, except it has several sets of mirrors ,frequently 8, centered around a common point.
If all people hold their mirrors at the SAME angle that would be good symmetry.

Also all people could be holding their mirrors at identical but wrong angles for max light return - this would also be excellent symmetry.

Now replace those people with mirrors with facets polished into a diamond.

 

[John P]

Kenny's explanation is excellent in terms of light performance angles and what may, or may-not occur in the diamond.

I wanted to clarify that the grading term "symmetry" found on lab reports has nothing to do with 3D cut-precision (the precise alignment of the flashlights in Kenny's scenario). In fact the lab-graded "symmetry" category only addresses two categories: (1) Facet-symmetry; the shape of the facets and whether they meet correctly, and (2) Proportion-symmetry; whether the main and break-facets are uniformly indexed, crown to pavilion, whether the culet on-or-off-center, the table is centered or tilted, etc.

In short: The "symmetry" grade as it appears on a lab report gives NO information regarding performance of the diamond. There is an old wives' tale claiming that EX Symmetry (lab report) naturally implies 3D cut-precision, but this is absolutely false, particularly in fancy shapes (see the linked thread below).

This is why I prefer the term "cut-precision" when talking about 3D optical symmetry (aka H&A for rounds), a subject which admittedly comes up on Pricescope far more than in casual trade chat. The fact that many tradespeople use the term "symmetry" interchangeably for lab-graded 2D sym or non-graded 3D cut-precision is confusing, as it can imply that some kind of 3D judgment is being made by the labs when it is not.

For anyone interested, four posts down in the linked thread are examples of diamonds(round brilliant) which all received "EX" or "Ideal" in lab-graded symmetry but have radically different levels of 3D cut-precision. In fancy shapes the differences can be far more exaggerated.

https://www.pricescope.com/communit...w-excellent-very-good-polish-symmetry.143434/

 

[Karl_K]

To add to John's excellent post.
A lower lab symmetry grade could have zero influence on light performance or it could mean that a facet is off enough to effect light performance.
It all depends on which facet/facets are affected and in what way.
It is much more common in fancies to have lower symmetry grades(under vg) that dont effect performance than rounds in my opinion.
It is a long explanation but basically many fancy designs have some facets at angles such that a vg or better polish is hard to obtain while making vg/ex lab symmetry grades due to the nature of the rough.
Modern tooling has made it easier to deal with these problem facets but the problem still exists and a cutter can chase a ex/ex polish/sym to the point they impact light performance by over cutting facets to less than optimal angles.
I at one time thought that g/g pol/sym was just cutters being sloppy and that is sometimes the case but at times a better grade is not possible.

bottom line its a case by case decision if there is an impact.

 

[Cy2009cy]

Woo, you guys are so knowledgable. Thanks a looot!
so for a fancy cut diamond, i guess the best way to determine if the lower symmetry affects light performance is see in person, right?
But if there is no way to check out by person, is there any way to figure it out?

 

[Karl_K]

is there any way to figure it out?

pictures, aset, video and the opinion of a trusted pro looking at the diamond who has a reputation for selecting well for that cut.

 

[Cy2009cy]

is there any way to figure it out?

pictures, aset, video and the opinion of a trusted pro looking at the diamond who has a reputation for selecting well for that cut.

Karl, would you like to take a look at the third picture(OMC) of my another thread and give some comments?
[URL='https://www.pricescope.com/community/threads/tough-tough-choice-please-help-to-choose-a-cushion.159072/']https://www.pricescope.com/community/threads/tough-tough-choice-please-help-to-choose-a-cushion.159072/[/URL]