High-bounce balls, often called superballs, are made of synthetic rubber. The original Super Ball, sold by Wham-O starting in the 1960s, used a hard polybutadiene rubber blend called Zectron, invented in 1964.
That material gives the balls a coefficient of restitution of about 0.92, so a superball dropped onto a hard, level surface bounces back nearly to the height it fell from. Older rubber balls, such as the Spaldeen, bounce noticeably less.
Source: Wikipedia: Bouncy ball