Plastic Rubber Jewelers Hammer
This lightweight 9-1/2" Plastic Rubber Jewelers Hammer features two different heads, one is a hard yellow plastic and the other is a soft black rubber.  The plastic head helps avoid the type of marring that a steel hammer might cause.  The rubber head is helpful for much more delicate work.

The head measures approximately 20mm in diameter (without the rubber heads).
The rubber replacement heads slightly taper to approximately 16.5mm in diameter.