I can't type special characters into the forum software so look at the Fresnal calculation here https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html to get the numbers for diamond. One thing about this though. Diamonds have dispersion and total internal reflections at certain angles. This material works more like the little speckles we see if you fake dispersion by adding each RGB channel together at a slightly different IOR and then mix the whole thing together with a clear glass material. That will provide the little coloured refraction speckles you expect to see. Search for BagginsBill's thread on diamonds and dispersion here on Rendo.