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Suppose we have a material which has a very, very high refractive index, say 30-50. The critical angle for this material would be very small (1.5-2 degrees). Will the said object be even visible at all, given that most of the light that enters it would not be able to come out again, at least until a few thousand reflections? Can anyone imagine/describe what it would actually look like?

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What makes diamond so attractive (at least to some who have bought into the marketing craze created by the de Beers cartel) is not just the refractive index but the dispersion (aka "fire"), which is among the highest of all gemstones. It's also important to the quality of a diamond to be as translucent as possible, especially for the larger stones and the cut is of extreme importance. A poorly cut stone with the wrong facet angles does not produce the sparkle that is considered a necessary quality criterion. In other words: a jewelry diamond is more than just its material properties, it's also an optimized optical system.

As a consequence for your hypothetical material it would be important to have extremely low absorption, otherwise there would be little if any light coming out. I don't know how dispersion would play out and what the ideal "cut" would have to look like. If anything that sounds like a non-trivial optics problem.

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  • $\begingroup$ You avoided answering the main question and instead talked mainly about diamonds. I would like to know more only about the "hypothetical" material. $\endgroup$ – user117913 Sep 9 '15 at 13:22
  • $\begingroup$ What the hypothetical material would look like depends on the shape of the hypothetical material and its other optical properties, which you haven't specified. You got one thing right, most of it would look black with a few specs of light around sharp edges and corners under the right illumination. It would probably be totally boring. $\endgroup$ – CuriousOne Sep 9 '15 at 13:25
  • $\begingroup$ What if it's a cube or a torus? $\endgroup$ – user117913 Sep 9 '15 at 13:29
  • $\begingroup$ Actually... there is an easy way to visualize this! Get yourself a ray-tracing software like Povray povray.org and make a cube with a material of your choice. Rotating cubes of glass are typically included in the examples and all you have to do is to re-define the materials in the scene description file. $\endgroup$ – CuriousOne Sep 9 '15 at 13:35

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