contrast or pattern in diamonds

[Sagebrush]

Reading the tutorial information on diamond cut. Excellent, by the way. Your discusssion posits two causes of pattern in diamond, light leakage and the viewers head. What about the role of off-axis refraction. Some of the dark areas in the faceup must be be caused by light that refracts at an angle away from the eye of the viewer this two will cause they eye to perceive areas of gray to black.

 

[Garry H (Cut Nut)]

Richard I presume you mean that light refracts away and does not come back?

if you do - then this is light that either comes from a light source in the environment, or it comes from a dark spot in the environment. But what you should consider is that where ever you see in a gem stone - it is making a window and mirror set that diirects your line of vision somewhere. Now it is not really even as simple as in that journal article - because in fact your line of sight will be partially reflected - around 15 to 17% most often, and then never does all the remaining light come out of the gem in one clean refraction reflection refraction.

But try this - think about your line of sight, rather than the light coming from a source.

Of course with colored gems we have double refraction and much higher absorption to worry about. Diamonds is easy Richard

 

[valeria101]

Just to add to the question...

From the public presentation of the GIA cut study (link) it seems that most of the analysis of diamond optics deals with a relatively restrictive range of in-comming ray angles - those said to "count most" in the scheme below.

(cited from GIA article)

Staring from a steep angle like this, it would take a shallow pavilion angle (less than 40 degrees) to reflect light out of the visual range of the viewer, unless they are very close to the stone. And if the viewer is very close, "head obstruction" plays out anyway...

At least, this is the excuse I found for the same question when reading GIA''s presentation initially. The restrictions of their model may be the most popular among cut standards, but not the end of the story

Hopefuly this thread will clarify what there restrictiosn leave aside.

 

[strmrdr]

All the current studies have a fatal flaw in that they dont reflect real world on the finger conditions.
Head shadow rarely comes into play on the finger.
Off axis source and return while tilted are much more important than any of the studies give credit too.
So trying to get too much out of it compared to the real world is a exercise in frustration.

my 2c

edited to reflect a view closer to what I feel about the issue.
Was too harsh the first time.