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.