Adamantium is a virtually indestructible man-made steel alloy which does not occur in nature and whose exact chemical composition is a United States government classified secret. Adamantium is not an element: its properties do not qualify it for any know space on the Periodic Table of Elements. Rather, Adamantium is a series of closely related compounds of iron created through a secret process discovered by the American metallurgist Dr. Myron MacLain.

MacLain began experimenting with the process that created Adamantium as a young scientist in the employ of the United States government in the early 1940s. Assigned to create a super-metal with which to build tanks, MacLain labored for month, experimenting with various iron alloys. One of his experiments utilized the rare meteoric ore now known as Wakandan Vibranium. He tried to fuse the Vibranium to the iron alloy numerous times without success. Then one night when he dozed off, some as yet unknown factor entered the process, and permitted the fusion to occur. Upon discovering his success, MacLain poured the molten metal into a disc-shaped mold. The disc, once solidified, has proven to be the most impervious object ever created on Earth. MacLain turned the discover to the government and it was given to Captain America to use as his shield. Neither MacLain nor anyone else has ever been able to discover what was the x-factor that entered the process, or has been able to fuse Vibranium with another metal. (The unknown iron-Vibranium alloy of which the shield is composed resembles True Adamantium, although Adamantium itself contains no Vibranium,)

Over the following decades MacLain experimented, attempting to duplicate the process that create the shield. Finally, in recent years, he succeeded in developing the process by which the substance known as True Adamantium is created. True Adamantium is nearly as strong as Captain America's shield, and is, for all practical purpose, indestructible. The degree of impermeability varies directly with the thickness of the Adamantium. A direct blow from Thor's hammer, conveyed with the thunder god's full strength, will slightly dent a solid cylinder of True Adamantium. A sufficient mass of Adamantium could survive a direct hit from a nuclear weapon.

Adamantium is created through the mixing of certain chemical resins whose composition is a United States government secret. For eight minutes after the resins are mixed, the Adamantium can be molded into a particular shape as long as it is kept at a temperature of 1,500 degrees Fahrenheit. After this brief period the process of creating Adamantium is completed. The extremely stable molecular structure of the Adamantium prevents it from being molded further, even if the temperature remains high enough to keep it in liquefied form. Only a device celled a Molecular Rearranger can alter the form of hardened Adamantium.

The United States government has shared the secret of Adamantium's composition with certain of its allies, and the secret has also fallen into the hands of various criminals, such as the Constrictor, who has made special weaponry from Adamantium. Wolverine possesses artificial claws made of True Adamantium and his bones have been fused with Adamantium in an unexplained manner so as to greatly increase their tensile strength. The outer portion of the robotic body of Ultron is also composed entirely of True Adamantium, although Ultron's inner mechanisms are not.

Adamantium is extraordinarily expensive to produce. Therefore, certain parties who wish to use Adamantium on a large scale have resorted to utilizing a similar but somewhat weaker compound similar to True Adamantium called Secondary Adamantium. Although Secondary Adamantium is still far stronger than even titanium steel, it can be destroyed by sufficiently great amounts of force. Blastaar's energy discharges and the vibratory force created by the Overkill Horn have both proved able to destroy Secondary Adamantium. The sentient computer known as F.A.U.S.T. had a casing made of Secondary Adamantium which Thor and Iron Man managed to destroy. The Red Skull and the original Hate-Monger had a dome of Secondary Adamantium constructed to protect their island fortress but the dome was destroyed by the vibratory force of the Overkill Horn and by powerful incendiary bombs.

First Appearance: AVENGERS #66.