The best answer so far is Vibranium. Based on the novelization of Iron Man 2, as well as the fact that it's the first thing that pops up if you google this question. Just because it's proven to have already existed in Captain America, and much later in Black Panther, doesn't mean Tony couldn't ave re-synthesized it in Iron Man 2. Howard Stark says in Captain America that the shield was made with the only known vibranium to the modern world, so it makes sense that he would try to synthesize it himself.

The best answer so far is Vibranium. Based on the novelization of Iron Man 2, as well as the fact that it's the first thing that pops up if you google this question. Just because it's proven to have already existed in Captain America: The First Avenger, and much later in Black Panther, it doesn't mean Tony couldn't have re-synthesized it in Iron Man 2. Howard Stark says in Captain America: The First Avenger that the shield was made with the only known vibranium to the modern world, so it makes sense that he would try to synthesize it himself.