Gorilla Glue can certainly help you out here! 

I've attached shells, buttons, metal embellishments and wooden decorations to terra cotta pots using Gorilla Glue.  The biggest thing to remember is clamping the items while the glue dries.  I've found that small clamps or even tape can be really useful to secure items around the edge of the pot!  Good luck and keep Gorilla posted by send any pictures of the your project to the Tough Stories section.