Add patterns to the surface of clay using texture sheets. These are flexible and will work on both flat and curved surfaces.
FIMO Texture Sheets - Professional quality. Transparent for easy positioning. Sets of 2.
Sculpey Flexible Texture Sheets - Detailed designs on thin and flexible sheets for use with polymer clays
Makin's Clay Texture Sheets - Easy to use to create textures. Sets of 4 sheets.
Texture Stamps from Lisa Pavelka - Select from the complete range of texture stamps presented in 19 sets of 2. Extra deep to create finely detailed designs in polymer clays and other media. See on YouTube.