Palladium

Palladium is an element. Tony Stark used it for his arc reactor, but it eventually began to poison him. He solved this problem by creating a new element, along with a new arc reactor.

Iron Man 2
Tony Stark was using palladium as the core for his arc reactor, but it was increasing his blood toxicity. After searching for every known element, he still could not find one that would not get destroyed, so he had to remain with palladium (which lasted the longest, compared to the others).

Natasha Romanaff, aware of this problem, informed Nick Fury of his condition. Fury provided Tony with some of his father's old thing. He discovers through a hidden message an element that could fulfill his needs. He proceeds to re-create this new element (using the molecular structure provided by his father) and creates another arc reactor that cures his palladium poising. He continued to use that reactor and disposes the old one.