Vibranium

"No! No! That's just a prototype." "What's it made of?" "Vibranium. It's stronger than steel and a third of the weight. It's completely vibration absorbent." "How come it's not a standard issue?" "That's the rarest metal on Earth. What you're holding there? That's all we've got."

- Howard Stark and Steve Rogers

Vibranium is a very rare element that was first discovered during the early 20th century. Vibranium is what Captain America's shield is made of.

It is from the African country Wakanda.

History
Steve Rogers was taken to a U.S. Army facility. He entered Howard Stark's workshop where Stark presented him with different types of shields on the table. Rogers was intrigued by a silver circular shield lying on a shelf. Stark informed him that it's lighter than aluminum and stronger than steel. He also added that the shield was all they had of the mineral.

In 2012, the Bus, a SHIELD specialized aircraft, included an interrogation room with walls built from a silicon-carbide vibranium alloy, meant for superhuman prisoners.