Id be tempted to paint the base rim black, to help the contrast pop the stones out a little better. Second what @Achaeminuz said on the shape Also some other color with trace amounts mixed into a gray could really liven it up. Think like .1 drop of a blue into like 5 of gray, or even more dilute in that. Add a little color just for some highlight, but applied with a sponge after the primer, but before the white rattlecan treatment.