The metal adamantium was first introduced in X-Men #194 (March 1980) as part of the storyline "Weapon X".
This alloy, created by the scientific division of the Weapon X program, is known for its indestructibility and is often used to augment the strength and durability of mutants or super-soldiers.
Pro tip: Adamantium's first appearance was in a story where it was used to enhance Wolverine's skeleton, making him nearly invulnerable.