Well the greatest problem with getting a diamond look good is as follows:
a diamond happens to work alike a prism, it total reflects light by bouncing it back to sender it also has some in purities so it eats light passing tough, now this is tricky, since shading models as a general rule dont do:
Now atmospheric effects such as the impurity they often do make.
So this means you will need to probably build the shading networks yourself, total refection is easy but chromatic aberration is just downright insane on your render times.
Now then you have a additional layer of complexity because you need to actually get right color levels for the reflection to get right exposure.
Nothing of this is very hard just have to be quite in tune with how physics work to get it correct. As you need to be able to stage tests to get the desired effect.
try using a photon mapping renderer (mental ray) to get more physically correct light
Doesn't help the shader need to be fixed for this to help, But yes you will need that once you get the shaders working to get he caustics going.
There is a alternative way. and that's to lie and cheat with simpler materials.
PS: As for the references, the blue MIGHT be absorption talking.
PPS: also the metals can not rely just on the env map they also need to reflect the object itself.
way too much PS:es: ambient occlusion is not applicable of these materials as a strategy, because they are highly polished and have VERY small diffuse components.