The Wham-O Super Ball is made of a Zectron, which contains the rubber polymer polybutadiene and sulfur.
"By combining polybutadiene with sulfur it was discovered by the ball's inventor, Norman Stingley. Like the adhesive behind Post-It Notes, this new rubber compound wasn't an intentional invention, but the material's ability to rebound 90 percent when greatly compressed led Stingley to believe he had a hit toy on his hands. And, of course, he was right."
from wikipedia:The name "Olmec" means "rubber people" in the Aztec language. the Aztecs and Mayans created a non-vulcanized rubber by extracting natural rubber latex from the Hevea brasiliensis trees in the local forests. They formed the latex into balls, and played the Mesoamerican ballgame with the resulting bouncy balls.