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 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 by researchers during the early 20th century. Vibranium is a nearly indestructible metal, which makes it highly sought after. Most notably, Captain America's Shield is made completely out of vibranium. The metal has only been found in the African nation of Wakanda.

Captain America's Shield
"The most versatile metal in the world, and they used it to make a frisbee."

- Ultron

In February 1940, Howard Stark presented vibranium in Los Angeles, a metal Stark Industries researchers discovered in Africa.

After successfully proving himself as a soldier in the battlefield, 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 to adopt as his main weapon as Captain America. Rogers was intrigued by a silver, circular shield lying on a shelf. Stark informed him that its unique vibranium composition makes it lighter and stronger than steel, and nearly indestructible. He also added that the shield had been forged out of their entire vibranium stockpile.

CXD 23215 Airborne Mobile Command Station
The Bus, a specialized S.H.I.E.L.D. aircraft, included an interrogation room with walls built from a silicon-carbide vibranium alloy, meant for superhuman prisoners.

Toolbox
S.H.I.E.L.D. Director Nick Fury used vibranium to make the Toolbox, a compact digital storage device that holds a collected database of classified S.H.I.E.L.D. files, and additional information, privy to any acting Director of S.H.I.E.L.D. He gave this Toolbox to Phil Coulson after making him the new director, telling him that the toolbox would make help Coulson rebuild S.H.I.E.L.D.

In order to find the Toolbox Alphonso Mackenzie placed a scanner programmed to find anything with vibranium in the engine of little Lola. With the information of the scanner Bobbi Morse was able to locate and take the Toolbox to Robert Gonzales.

Since Phil Coulson was unwilling to open the Toolbox for Gonzales, the device was given to Anne Weaver to crack. Unable to crack it herself, she asked Leo Fitz to help her. During his research on the Toolbox he was unable to find any ports or external power sources in the pure vibranium box. He also found that there were pores in the vibranium which were constantly shifting.

In their last hope to open the vibranium box S.H.I.E.L.D. flew in a state-of-the-art, next-gen fiber laser from the Iliad to the Playground to cut through the vibranium and finally figure out the truth.

Ultron's Body
Ultron made plans for global extinction since its very creation, which consisted of using a combination of vibranium and Chitauri technology to create an extinction level event. To that end, he sought out Ulysses Klaue, who had smuggled large amounts of the metal from Wakanda. Either buying or stealing a large amount of it, Ultron used the metal to start building a new, synthetic body for himself, with the assistance of Helen Cho's Cradle. This body would become the Vision. Though he lost the body to the Avengers, Ultron had enough vibranium left over after creating his doomsday device to build a completely new body for himself.

Capabilities
Vibranium is an extremely rare element that possesses unique attributes. In a pure mass, vibranium forms a solid metal that is stronger than steel, but only a third of the weight, as well as being completely vibration absorbent. This makes the material nearly indestructible, as any attempt to physically damage the material will be negated on a molecular level as mechanical/kinetic energy will be absorbed instead of breaking any bonds in its molecular matrix. In general, this means that vibranium can absorb a wide variety of physical impacts, including high-caliber bullets, without taking any damage or weakening.

Vibranium also reflects kinetic energy. Captain America's vibranium shield for example, was able to reflect an impact from Mjølnir in a form of a shock wave. It also deflected high caliber bullets and Iron Man's repulsor beams by having them ricochet into enemies. Captain America's shield has also been known to be thrown at a high velocity with out being affected by wind resistance and is able to ricochet off of multiple surfaces.

However, despite its near-indestructible nature, Vibranium's durability has its limits; the energy projected by the Infinity Stone in Vision's forehead combined with Iron Man's repulsors and a lightning bolt from Mjølnir was able to slowly melt away Ultron's Vibranium outer shell.

Vibranium has magnetic properties; Bobbi Morse used her battle staves to procure the Toolbox from its container with magnetism.

Trivia

 * In the Iron Man 2 novelization, the element created by Tony Stark to replace Palladium in the Arc Reactor is called Vibranium. The subsequent release of Captain America: The First Avenger retcons that information.