Hard Rubber. A firm, inelastic, nonmetallic composition once used for medallic items, not replaced by plastics. The hard rubber is formed from natural rubber (terpene hydrocarbon) treated with sulfur – the harder the rubber the greater the amount of sulfur present during the initial heating. When left in natural state hard rubber is blank (called ebonite); it can also be artificially colored by dyes (called vulcanite, from the vulcanized process). See illustration, product medal.