What is the best material, texture and bumps I can try to renders dimonds and jewellery.
And studio toning I can apply in keyshot to get best realistic results
Probably the best tutorial is here in their official jewelry rendering webinar:
As far as materials go learn what each parameter in keyshot actually means and then try recreating the optical properties close to the real ones (just google for any database and you should be sweet for refraction indexes and etc).
Also if you haven't watch more of their webinars, it will allow you to increase work flow and get an understanding of how things work in there
I have simple used the gold material and GEM material available in Keyshot library, but the environment is a custom HDRI, which gives it more realism, at last post processing was done in Photoshop