Hi,
I tried to create a few shaders without any internet help, that took quite some time 
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I wanted to create a prism and diffract light. Then I modified some values to create jewels.
Just before posting, I looked at what some of you made and we had some similar ideas, so some of my materials might resemble what others have already done 
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Prism is diffracting light ! Yay !
The black thing is a box with transparency for camera rays, so light is actually entering the box through the prism. I didn’t know how to show you the diffraction (with all the other materials) in another way 
(Check the blend file)
Blender file: diamond test.blend (1.7 MB)
You can play with light now, and create your own jewels, good luck !
Cheers !