importance of symmetry?

[atelier7]

there''s seems to be a lot of emphasis on the right proportions - especially crown and pavilion angles to try to select a diamond that is brilliant.

But how much of a difference does symmetry make to its brilliance?

What you consider a stone with "very good" proportions but "good" or "fair" symmetry?

 

[oldminer]

It seems that recent data may indicate that symmetry is basically a positive thing, but that in larger diamonds a bit of asymmetry may work to improve overall scintillation. I think symmetry is one of those things we can measure, but may not be all that terrifically important to how well satisfied your diamond makes you. Obviously, a diamond with a strange bulge on one side or a hump-back appearance will disturb the eye, but a little mismatching of facet shapes may not bother you at all. It will lower the grade to good or fair, but can you even tell?




Such a lower grade will lower the price of the diamond, but you might well find it a great stone and a better deal financially if you have a budget goal.


It is part of what needs to be measured to define perfection, but not all of us care about perfection as much as overall satisfaction.

 

[Anthonyd]

This is from a description on HRD grading. Here is the link for the complete article:
http://www.diamonds.icestore.com.au/diamonds-diamond_information.htm


Cut or make is the last of the 4 Cs. On the HRD certificate, two aspects of the cut are judged separately: the finish grade and the proportions. When examining the finish grade, attention is mainly paid to possible symmetry-deviations and facets. A good finish grade testifies of the workmanship of the diamond polisher.

The proportions, the relationship between the various parts of the stone, determine the fire and brilliancy of the diamond. If the proportions are not optimal, undesirable visual effects may occur. The main symmetry-deviations and the proportions are first judged on the basis of objective, measured data. For this purpose, a Dia-Mension system is used: with the aid of a camera a large number of measurements are taken, after which these are interpreted by a computer. The results are then checked by comparing them to the grades given by the HRD experts on the basis of visual examination with a loupe.

"Cut" refers to the overall description of the proportions on the one hand and the finish of the diamond on the other hand. The proportions are determinative for the brilliancy and the fire of the diamond, while the finish represents the extent to which deviations occur from the symmetry and other characteristics of the cut. The description of the cut on a certificate is split into a separate description of the proportions and of the finish.

Finish Grade
grade definitions
very good no or negligible symmetry-deviations and/or finish faults
good the symmetry-deviations are hardly perceptible
medium the symmetry-deviations are perceptible
poor the symmetry-deviations are significant
The four main symmetry-deviations are:
- variation on the average diameter (for brilliant-shapes)
- variation on the average crown height
- deviation from the central position of the table
- deviation from the central position of the culet

 

[Anthonyd]

The rest of the article:
http://www.diamonds.icestore.com.au/diamonds%20-%20Ideal%20Cut.htm

Proportions
The brilliancy and fire of a diamond are largely determined by the angles of inclination and proportional relations between the various parts of the stone. If these are not optimal, specific undesirable effects can occur. When grading the proportions of a polished diamond, the main issue is therefore to evaluate if, and if so to what extent, these effects occur. The most important effects that can be perceived when observing the stone perpendicular to the table are:

fish eye :
the reflection of the girdle is visible through the table
black table reflection : the reflection of the table is so large that the table appears to be almost completely black
culet visible in bezels : the stone shows an abnormal amount of scintillation, due to the culet and the surrounding facets being visible through the bezels
window : the light falls through the stone and leaves it at the bottom without any reflection
single cut : the diamond looks as if it has less facets than it actually has because the difference in the angle of inclination between the facets is too small (no sharp edges)






The list of effects above is not exhaustive; there are other proportions -related phenomena which are considered to be undesirable, for instance the diameter being too small in proportion to the total depth, making the stone appear smaller than its actual weight.


For the brilliant- shapes, next to the actual values an appreciation can be given :very good / good / unusual The measurements of the different parts of the stone can be a useful aid in determining the proportions grade, since there exists an obvious relation between these data and the appearance of undesirable visual effects.

 

[oldminer]

The question which began this thread is how much does symmetry affect brilliance?




All the other info aside, does anyone have a different view than my own on symmetry. A review of the entire art of diamond grading does not address the asked question in a way that a consumer might learn something or that there might be different opinions on this specific matter. symmetry/brilliance.

 

[Blueman33]

As a non expert, I looked at stones with VG and VG (GIA) symetry and polish and stared at them till I was bored and couldn't tell the difference in them and EX EX stones. Looked in a loup, couldn't tell the difference.

While I found a stone with ID ID (AGS), I would have been perfectly happy to get a stone with a grade lower in symmetry (and polish) to save some dollars.

Few months ago, someone posted IS images of either a VG or G symetry and it had perfect arrows.

So in this layman's opinion, an ID or EX rating for symetry is for a perfectionist more than for visible performance.

 

[Anthonyd]

According to the article that I linked above, Proportions are what affect fire and brilliancy the most. Does anyone know if this is 100% true. That article is from HRD.

 

[JohnQuixote]

In a discussion of “Symmetry” it will be of use to note the difference in “meet point symmetry” which is what the labs are grading, and “physical symmetry” which has a few more elements.

Meet Point Symmetry

A: Facet junctions meeting equally (seeing if the lines are clean and there is no misalignment).
B: The crown in relation to the pavilion (making certain all of the main facets are aligned - that there is no “twist”)

Physical Symmetry

1. Physical symmetry results from a detailed indexing of the facets (that crown and pavilion facets appear in their correct place on the ‘clock’).
2. Another aspect is how flat facets are relative to their opposites. This has been a subject of study by Brian the Cutter for many years. He is terming it “Yaw” of the facet, and brought it up in discussions after his presentation at the IDCC (the International Diamond Cut Conference last April in Moscow). This could be the subject of a new thread…
3. The third factor is variation between crown and pavilion angles: As we have mentioned (thread here) it is acceptable to have some variation (subject to limitations!) as long as opposite facets average correctly. Some might consider this as an angles/proportions issue, but it is directly related to physical symmetry, as the question here is opposite facet relations, averaging and resultant light interaction. For that matter, we’ve observed that a slight variation in opposite angles can indeed improve overall scintillation and contrast (Dave alluded to this).

Optical Symmetry

Optical Symmetry is the observable result of overall physical symmetry, but is not an exclusive result of meet point symmetry (which is what the labs grade). Though meet point symmetry is physical in a way, the labs do not look at how each individual facet performs or interacts with other facets; They check the junctions and crown/pavilion relations. A stone can receive a grade of excellent/ideal in symmetry but may still have indexing issues (bottom clock in relation to the top clock), “Yaw” of the facets and extreme angle variations.

The only way to observe physical symmetry is through a Hearts and Arrows viewer (though IdealScope works for this to a degree). Brian’s presentation at the IDCC was about how Hearts & Arrows should be graded and touched on pertinent points, above. Garry Holloway’s site has a complete transcript of the IDCC proceedings for anyone interested, linked here.

As to the question:

There are people who feel that diamonds are beautiful without orderly symmetry. Our position is that symmetrically precise alignment of the mirrors optimizes the return of light. Slight angle variations in opposite pavilion mains enhance the effects of scintillation and contrast as long as there is no “yaw” to the facets and the opposites average correctly. In short, we feel that overall symmetry does improve a diamond’s visual performance.