Is ''Optical Symmetry'' ''Needless Perfection'' ???

[michaelgem]

Bruce Harding posted the following response concerning a question on the USFG forum about a Hearts and Arrows scope.

The purpose of a ''Hearts & Arrows'' scope, ''FireScope'' and
''Ideal Scope'', is to show two things:

1) Light-dark contrast between certain facets in a controlled lighting environment, which can be achieved only by maintaining certain pavilion and crown facet SLOPES;

2) Perfection of cutting geometry - like kaleidoscopic symmetry - especially with regard to INDEX ALIGNMENT of crown & pavilion facets.

These scopes are used extensively in the diamond trade, primarily on round brilliant cuts, as a way to indicate quality of cutting geometry.

They might be a good tool for judging the cutting quality of colored stones, with regard to perfection of index symmetry.

Bruce Harding
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I supported his post with the following comment:

Obtaining ''optical symmetry'', which is easily evaluated with FireScope like devices, is much harder to achieve than conventional measures of craftsmanship. So much so that diamond cutters who haven''t the time or the skill and do not see the need for this degree of optical and mechanical precision, have called it "needless perfection".

For many this may be true, but how many educated and informed brides do you suppose think their diamond was cut needlessly perfect.

Here is a side by side photo of two American Ideal cut diamonds, one displaying ideal ''Optical Symmetry''.

Michael Cowing
ACA Gemological Laboratory
http://www.acagemlab.com

 

[Garry H (Cut Nut)]

Michael did you see some threads where Marty Haske was defending EightStar and Sergey raised the issue that optical symmetry could be bad in cases of larger stones - probably over 7ct.

I always find it funny that people seem to want extra facets - yet you get a similar effect with lessor symmetry.

I also still hold that a slightly shallower proportions set can save you if the symmetry is not so hot. Call it a stone in the sweet spot, like say 57% table, 34 crown and 40.5 pavilion.

 

[Mara]

I assume the right hand stone is the one with optical symmetry? I don't know...it looks a little TOO perfect to me? The left side stone looks more faceted? Interesting..while I like the arrows in the right hand stone being very visible, the square looks so visible as well. I almost like the left hand stone better!

 

[Garry H (Cut Nut)]

Odds on the RHs is an 8*.

The other stone has longer lower girdles - it would appear "blockier" too if it's were shorter too.

 

[michaelgem]

Garry,

Re:
On 11/6/2003 3:09:43 AM Cut Nut wrote: Michael ,
I always find it funny that people seem to want extra facets - yet you get a similar effect with lessor symmetry.

Right on. Asymmetry contributes to a greater number of smaller reflections giving the appearance of a diamond cut with extra facets. This can increase scintillation, but it may decrease the perception of fire.

This is why I said to Iiro:

Briefly, a non-optically symmetrical American Ideal like the one on the left has more numerous and smaller reflections of brilliance and fire, while the optically symmetric American Ideal has larger reflections in the face up normal viewing position due to the precise facet alignment.

Notice the large flash blue fire from the symmetric right diamond's mains. The greater break up of the reflections occasioned by the lack of optical symmetry can contribute to greater scintillation, and at the same time reduce the perception of fire. Especially in smaller sizes, the broken up fire may be too small to register as color to our vision or it may add back to white with visual merging of adjacent reflections like the rgb color guns of a monitor.

Sergey's observations raise the question as to what is the best range of reflection size for observation of brilliance, fire and scintillation? Does a very large diamond need more than 57 facets or asymmetry to give the appearance of more facets? And is the converse true? In small diamonds is there a greater need for optical symmetry or less than 57 facets for the best brilliance, fire and sparkle in very small diamonds?

The answers are complex and differ from brilliance to fire and scintillation. Ultimately, individual taste for the balance between brilliance, fire and scintillation and how these beauty factors are presented and sold to the public will influence the answers and the outcome of future diamond design.

Michael Cowing
ACA Gemological Laboratory

 

[michaelgem]

----------------
On 11/6/2003 4:27:35 AM Cut Nut wrote:

Odds on the RHs is an 8*.

The other stone has longer lower girdles - it would appear 'blockier' too if it's were shorter too.----------------

Garry's right that broader flashes of fire and scintillation in Ideals like the one on the right are due not only to perfect optical symmetry, but also to the larger mains occasioned by slightly smaller, shorter halves (lower girdle facets).

The broad flash fire and 'bang-bang' sparkle that appeals to many in the Old European and Old Mine cut diamonds are due to shorter halves and broader mains.

The length of the halves heavily influences fire and scintillation. Longer, larger halves that result in thinner mains yield greater but more pinpoint sparkle, while smaller halves and broader mains yield broad flash fire and sparkle.

Slight facet alignment asymmetries, such as are present in the left diamond, make a contribution to the reflection pattern similar to thinner mains and longer halves.

Ultimately, it comes down to a matter of taste. In opal, for example, broad flash fire is valued more highly than pin-fire.

Michael Cowing
ACA Gemological Laboratory