I use two brushes for my gems, a 1/0 (or simply 0) and a 5/0 with a very fine point.
I actually do them a bit backwards, starting with the lightest shade of the gem and using the *side* of the 1/0 paintbrush's bristles to coat the gem. Then I use the next darker shade to make a slightly smaller oval, and then the darkest shade for the smallest oval - essentially concentric rings/ovals.
Then I dilute Skull White 1 part with water 3-5 parts and use the tip of the 5/0 brush to make a tiny dot on the darkest oval and a small line across the lightest oval on the bottom. Looks pretty decent!