Roman Boichuk shows how to render diamonds and jewelry to a highly realistic level using Octane in Cinema 4D.
PanRomanCG (Roman Boichuk) explores how to create the most realistic diamond shaders when creating jewelry in Cinema 4D. The tutorial shows a simple way to make the diamond shaders in Octane.
More on Diamonds and Octane.
Another tutorial that deals with diamond shaders and the light effects that make them work was one by SilverwingVFX that covered working with aberration, dispersion, wavelength colors, and refractive material theory. Visit that tutorial here.