Typically, HDR Studio to create new or edit existing HDRI files, then use the HDRI file in favorite renderer that supports such. However, HDR Studio can be used standalone too, to an extent.
Keyshot can edit HDRI files as well. Not sure if RhinoCycles has, or will get, such capability? @nathanletwory
I’m not a jeweler… but play with the HDRIs. Find (or create) one that seems suitable and is particularly applicable. Depends on scene too. The more in depth and intense the HDRI, the greater the resulting influence.
Same advice as to @sworkman. Find, edit, or create an HDRI that is suitable and applicable to what you do. You’ll get the hang of it and likely reuse favorites once tweaked just right for you. Hint - if too intense, simplify HDRI, or remove/tone hot spots.
You can control darkness with either the intensity of your HDRI, or other lighting, or your material specifics, etc, etc.
I simply post-processed your rendering in PS, but results of tweaking at render will be similar.