Exactly what sort of 'look' are you trying to achieve? Very simple adjustments to either the materials or the environment (HRDI) can product significantly different results.
There is already a good tutorial on this website for jewelry rendering:
I agree with Alan that HDR changes could be all you need here. I would also try using the HDR color for the skylight as well by checking "use environment settings" in the Luma server. You can find a bunch of free HDR environments on www.Food4Rhino.com if you weren't aware of the site.
The best place for tutorials is in the 'Tutorials' section on this site at the top. For jewelry specific scenes, the link posted in this thread is also useful. Let me know if there is something particular you're looking for.