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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sun Nov 22, 2020 1:32 pm 
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I think I may not have been clear about my point of the question Justin.

By the conditional statement "if you have enough depth", I meant that you would not have to exceed the critical angle and create a window.

The R.I. of the material determines the critical angle.

The critical angle basically only sets the minimum angle without a window (sort of, more on that later), not the optimal angle for the cut.

This is demonstrated by comparing Quartz with Diamond. Quartz has a R.I. of 1.545, and a critical angle of 40 degrees. Diamond has an R.I. of 2.42, and a critical angle of only 24.5 degrees. These are pretty much at the opposite ends of the scale for gemstone materials. Yet, when doing the optimal RBC cut for either of these materials, the recommended pavilion angles are with in a degree or two of each other. Quartz being about 42 degrees, and Diamond being about 41 degrees. In general you will find that no matter the R.I. keeping that pavilion angles that are visible inside the diameter of the table to within the 45 - 40 degree range creates attractive and brilliant gemstones.

This is because Snell's Law is the dominant factor for optimum return of light once that light is internal to the stone. Snell's Law states that "the angle of reflection is equal to the angle of reflection". Just like the path of that a pool ball would follow.

Refractive index is a ratio of the speed of light in a material, compared to the speed of light in air. It defines how the light is bent or reflected when light traveling through air strikes a material at a given angle. In our case a transparent gemstone material.

Critical angle should be viewed as a cone. Light that strikes a surface at an angle within that cone will pass through the material/air interface (surface). Light that is outside that cone is reflected to another surface within the stone. We want as much light striking pavilion surfaces to be outside this cone, and as much light as possible coming back up through the crown to be within the cone so it exits back to viewers eye.

Stones cut in the standard brilliant or step cut patterns tend to form a "hard" window. That is because these cuts have the minimum number of secondary, tertiary, and so forth reflection paths. These cuts a very efficient in getting light back out to the viewers eyes in the shortest path traveled within the stone. . This is very good for brilliance. However, it creates a very hard window when critical angles are violated. (It is often not the best solution when it comes to managing color saturation, pleochroism, color zoning, and minimizing inclusions. All of these are also the responsibility of the cutter. This sometimes includes purposeful windows to enhance color on over saturated and color change material. But that is another discussion).

If you have a stone that is a bit thin, or you just want to save weight by carrying a larger bulge factor, you can create a "soft" window. You do this by cutting in a style that creates more secondary, tertiary, and on, reflection paths. This splits the light into as many reflection paths as possible within the stone. Making sure that the minimum of light strikes the surface of any facet within the CA cone. It also usually involves interlacing facet tiers like you do with Portuguese style cuts. Yes, this decreases the light being returned to the viewer from minimum reflection paths. But, it also reduces the amount of light leaking out from any surface and creating a window. This and the interlacing of facets, lowers the contrast between facets that are above and below the critical angle. Creating what I call a "soft" window. This results in a much more attractive stone that one with a "hard" window.

By choosing such a cut you can often close a culet by cutting below the critical angle by a few degrees without creating a hard unattractive window in a stone.

Experiment with this on some inexpensive pieces of quartz or a synthetic, and you will see what I am talking about.

I hope I have been more clear this time.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Sun Nov 22, 2020 8:59 pm 
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Yes I understand you completely. This is exactly as I teach it in my faceting classes at IGT. :D

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The views expressed here are the author's opinions and do not necessarily reflect those of his employer.


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Mon Nov 23, 2020 8:15 pm 
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Thanks for your input. More questions now: Can you be more specific as to what kind of cuts produce more light "bounces" and inner reflections compared to others? I am guessing cuts with simply more angles/tiers and facets. Does a steeper crown affect things? Also, as far as step cuts and brilliant cuts, what kind of stones are a better choice for each type? I think I have heard step cuts are better for darker stones, but I am not sure if that is just because many step cuts seem to have larger facets. Does dispersion and refractive index make much difference in choosing a step vs a brilliant cut?

Thanks again


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 Post subject: Re: questions on the nature of gem cutting and light renderi
PostPosted: Mon Nov 23, 2020 8:51 pm 
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Yes, more smaller facets, ,ore tiers on both the crown and pavilion even out the way light travels through the stone.

Fewer facets, and shallower cuts to darker stones. Step cuts often work best. Light absorbs color in direct proportion to the length of the line the light travels inside the stone. In dark material the shorter the path, the better. The shortest path possible is actually transmitted light, which is why over dark material is often purposely given a window.

In light material the longer the path the light travels the more color it absorbs, the more saturated the stone looks.

I don't thing that R.I. makes that much difference directly. However, it does make a difference when considering choosing a cut for a shallow piece of rough. This week I cut a Spessartine Garnet with a 1.80 R.I.. Normally as I said I try to keep my minimum pavilion angles above 40 degrees. In this case I was able to maintain the full width of the rough, and a decent bulge factor for yield and cut my shallowest pavilion angle just below 36 degrees. I choose a step cut pavilion, and brilliant cut crown. because this provides the easiest bulge management solution as the difference in angle between tiers doesn't affect either the shape of size of facets. It also let me easily cut the ends much steeper than the centers also increasing yield.

The crown has very little impact on brilliance, but has definite impact on pattern, and contrast.

For stones where dispersion is important, it usually is best to have high crowns, and small tables. Since dispersion mostly shows in light color materials and is hidden in dark materials, this works out well.


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