Sup Dakka.
Was just doing a little research, and I found an absolutely fantastic source of reference for this particular topic: painting gemstones.
It's 1 of those things that novice to intermediate painters are nervous about, but have no real need to be, ya know?
So, here's the easiest of easy examples of how a painted gemstone should work:
Here's amping it up a bit. This is where you really start getting the "Ooh... Aaah" feel from the gem:
Now here's the real deal. Different shaped gems demand different highlights, and this is a wonderful example of that very fact:
I didn't post this in the tutorial section, because I have in no way actively participated in teaching anybody anything. Just me, researching, finding good info, and passing said info onto my fellow Dakkas.
Enjoy