If this is for 3D printing, are we talking FDM or are we talking Resin? It kinda does matter.
The first thing I would recommend would be to make sure that your basis shape is correct. Looking at what you have compared to your reference, the shape isn’t quite right, and the top detail (with the celtic knot) looks to be far too small. So that would be my first step. Really study the shape, and get that right first.
As for the details:
For the Gem, don’t worry about trying to add physical details. It’s pointless, as it’s just chips and scratches, right? Sand paper in the real world can easily add that kinda wear and tear.
For the engravings:
Once you have your basic shape finalised, You can then simply model the details, starting with a plane and using super basic techniques (E to extrude, or simply CTRL Clicking two parallel vertices). Snapping and shrinkwrap will help you a bunch here. You don’t even have to give the model any thickness. Simply add a solidify modifier, (and sub surf modifier if you want it to be smoother) and slowly add your details.
If that sounds super complicated, then I would recommend getting to grips with the basics first.