CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: brilliance and fire in rose cut diamonds
PostPosted: Sun Jun 04, 2006 2:37 pm 
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Hi,

During the Saturday chat this weekend we were thinking about brilliance and fire in rose cut diamonds.
Ofcourse they are not the same for brilliants, but to what extend.
My logic tells me me that when cut at an angle (to the base) larger than 25% (corner a) and light coming from an angle, fair brilliance and fire can occur in these (unfoiled) rose cuts.
I would like to hear what everyone thinks about this logic of mine.

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PostPosted: Fri Jun 09, 2006 11:13 am 
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No takers? I gotta do everything myself around here.


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PostPosted: Fri Jun 09, 2006 1:02 pm 
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Ok, I'll dummy up,

You may effectively change the brilliance or scintillation on a rose cut by increasing the crown angle, but, I think it would have little effect on dispersion or fire.

The inherent problem with rose cut diamonds is their flat base. To much light is lost through leakage. Dispersion or increased dispersion isn't only affected by refraction but also the amount of time the light spends in the diamond which requires it to be reflected many times off the interior facet walls. The more this light is reflected the greater the spread of the spectral color componets and the greater the fire once is finally exits the crown facets.

Of course light stikes a diamond from every angle not just the angle as shown in the diagram. So you may make some gains in reflected light, but, I surmise little will be gained in dispersed light with the majority being lost through the flat base.

P.S. In the waste not want not world of diamonds, I think the rose cut was just a way to make use of shallow flattened rough or mackles. Not for beauty but for the added value a piece could bring that contained anything with the D word. Has anyone ever seen what amount to a beautiful rose cut diamond? In terms of it's interaction with light?


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PostPosted: Fri Jun 09, 2006 1:45 pm 
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Yes ofcourse most of the light would leak from the base regardless of the height of the rose. The amount of dispersion (from the reflected part) would not be affected though, only the spread of dispersion .. or am I missing a vital ingredient?

Do you think my calculations are good, i.e. angle "a" > critical angle may show descent fire?


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PostPosted: Fri Jun 09, 2006 2:32 pm 
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I think your diagrams properly represent your theory of an increased crown angle showing and increase of brilliance but not necessarily increased dispersion.

Both diagrams show a beam of light striking the crown facet perpendicular to the surface. this beam in both diagrams enters the diamond and slows down but does not refract. No refraction, no dispersion.

As the beam of light continues towards the base, the top diagram shows the light reflecting from the base and exiting perpendicular to the opposite crown facet. Again no refraction.

The lower diagram shows the same perpendicular entrance but falling within the critical angle and exitng the base.

Because there is no refraction in either example, I would say there is little effect on dispersion but a greater effect on brilliance in the top diagram.

One possible effect that may have a very subtle effect on the dispersion in the top diagram is polarization. Each time a ray of light strikes an interior surface of the diamond the light becomes more polarized effecting how much light is reflected or transmitted. this could effect the overall dispersion and brilliance as well. I just don't think there is enough mechanics happening within this rose cut fashioning to produce a marginal difference.


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PostPosted: Fri Jun 09, 2006 2:57 pm 
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Quote:
I think your diagrams properly represent your theory of an increased crown angle showing and increase of brilliance but not necessarily increased dispersion.

Both diagrams show a beam of light striking the crown facet perpendicular to the surface. this beam in both diagrams enters the diamond and slows down but does not refract. No refraction, no dispersion.


Very true, along that angle there is no refraction, so no dispersion can occur. But if I would shift the incident angle to the left a bit it would. In the top image the angle a is 45 degrees and the incident ray is at 90 to that. What if I would lower angle a to 40 and keep the incident ray as is?

Would that give me fire in theory? And is my "minimal 25 degree" angle a plausible concept?


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PostPosted: Fri Jun 09, 2006 7:04 pm 
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Well if you lowered the crown to 40 deg. and no change to the incident ray you would alter the angle of incidence.

The same would be done by doing nothing but simply turning the stone a few degrees.

That is the problem with static diagrams. They give no variables of movement, so your question becomes one of geometry as opposed to light play within a diamond.

Diamonds are at their best in motion. A static veiw just gives one reaction to one incident ray at any given point. The veiw of a diamond changes little in a static position. To best judge the reaction to light the diamond or the veiwers position should change. Then you will see the response to the variable cutting angles as light reflects or refracts in the changing light enviornment.

Not that your theory wouldn't affect some aspect to light response, just that the flat base is an inherent bad design for diamond optics.

Didn't Tolkowsky already figure this out? lol

Anyway it's a good lesson on light optics but, the subject stone has limited potential in my opinion to produce any spectacular improvements without the aid of a properly designed pavillion.

The question that really puzzles me is.....why some people purchase expensive diamonds without even looking at them. They just buy the paper and are satisfied it will meet their expectations because the certificate says so. They should have the certificate laminated and worn like a name tag on their first day of school.


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PostPosted: Sun Jun 11, 2006 2:34 pm 
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An interesting follow up read on this subject:
http://www.gia.edu/research/1383/2307/a ... detail.cfm

It's helpful in understanding the difference between dispersion and fire and what actually happens once a beam of light strikes a diamond under various conditions.


Last edited by JB on Sun Jun 11, 2006 2:53 pm, edited 1 time in total.

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PostPosted: Sun Jun 11, 2006 2:50 pm 
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And from another article:



[Footnote 6] This is true except in cases where the light beam strikes at a perpendicular angle. Even in these latter cases, the light beam will almost certainly start to spread into its spectral colors at the very next facet interaction. One light beam that would not undergo dispersion would be a light beam that strikes the very center of the table at a perpendicular angle, travels through the center of the diamond, and then exits the culet (also at a perpendicular angle). [back]


The footnote doesn't specify nor exclude any particular angle of the second facet interaction.









[back]


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PostPosted: Wed Jun 14, 2006 10:58 am 
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Thanks, nice articles.


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PostPosted: Wed Jun 14, 2006 12:53 pm 
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Your welcome,

I think the better answer to your original question is, "I don't know."

Certainly light play within a diamond continues to be a major field of study with expensive software, ray tracing equipment, lifelong experts still trying to unlock all the secrets.

As we read more on the studies of this, we see that basic principles of reflection, refraction and dispersion become a bit more complicated once a ray of light has entered a diamond.

Thousand of reactions occurring simultaneously with thousands of rays of light. Light refracting, dispersing, recombining, striking facet junctions and splitting the spectrum, polarization and on and on.

Glad no one is paying me to figure this out. I'd be pink slipped.

Your question seemed simple at first glance, but with further thought and considering that your diagrams would be actual faceted diamonds, three dimensional in a total light environment, my better answer is, "don't know." Well...except for the flat base part.


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