Fun Express Rubber Rock Bouncing Balls - (12 Pieces.) Rubber. 1 1/2" (38mm) Amaze your friends with these Rock-Shaped Bouncing Balls. All the cavemen at your bedrock party will enjoy tossing these bouncing rocks around the gravel pit. Add these unique bouncy ball toys to birthday party goody bags. Rubber. 1 1/2" (38mm)