i use sattelite city hot stuff glue. its a two part high strength glue. put the epoxy glue on one side of the broken piece, and spray a catalyst on the other piece and press the two together and it will bond and harden almost instantly. it can be sanded and polished. dries crystal clear. this is the stuff the granite guys used to fix a chip in my countertop. i bought it at a granite supply store but most hobby shops carry it also.