A setting around 35-55 is a good starting point if a subtle dispersion effect is desired. A low value will show heavy dispersion, and as you increase the value, the effect will become more subtle. A value of zero will disable the dispersion effect entirely.
This prismatic color effect can be used to create the “fire” effect often desired when rendering gem stones. The Abbe number controls dispersion of light as it is transmitted through the surface and produces a prismatic effect. This can be used with a low setting to produce a more imperfect/noisy result or a high setting to smooth out the noise/grain for a smoother frosted look. This is used to create a frosted glass look. Expand this parameter and you will see a samples slider. However, it also spreads out the light that is transmitted through the material. Roughness on this type of material will spread out highlights on the surface similar to what you see on other, non-transparent, materials. Without this you would see through to the bottom of the deepest ocean as easily as to the bottom of a swimming pool. This physically accurate parameter simulates the effect you can observe by looking at the color of the shallow water at a beach versus the deep blue of a deep ocean. A lower setting will show the color more in thin areas of the model, and a high setting will make the color faint in the thin areas. This slider controls the depth of the color selected in the Color setting, depending on the thickness of the part to which the material is applied. After setting a color in the Color setting, use Transparency Distance to make that color more or less saturated and prominent. A Table of Refractive Index for Gemstones can be found on the International Gem Society website. The default of 1.5 (the Refractive Index of Lapis Lazuli) is a general setting and will need adjusted depending on the gem you desire. This controls how much light will bend or “refract” as it passes through parts on your model that have this type of material. The amount of color you see in this material is highly dependent on the transparency setting as well.
When light enters the surface it will take on the color set here. This controls the overall color of the gemstone. The settings have been optimized to be relevant to rendering gem stones. The Abbe Number (dispersion) control is especially important for gem stone rendering as it produces the “fire” effect that is often desired. The Gem Material Type is related to the Solid Glass, Dielectric, and Liquid Material Type.
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