Adamantium is an extremely rare and valuable alloy of steel. The secret for producing it is a closely guarded one, and the cost is extraordinarily prohibitive. It's nearly as expensive as Vibranium or anti-matter on a per-gram basis.
True Adamantium is formed through a chemical process that creates a ferrous epoxy at 1500 degrees Fahrenheit. Once this epoxy sets, Adamantium becomes functionally indestructible. It requires heat levels that exceed the surface of the sun, precluding most blast chambers from being able to contain the heat.
Secondary Adamantium is slightly more common and cost effective. Known as 'Adamantium Steel', it is substantially stronger than most alloyed metals in current use. It is still quite expensive and lacks the inherent thermal resistance of True Adamantium.
Variant: There is only one known incidence of 'Proto-Adamantium' in the universe: The shield carried by Steve Rogers of Earth. Formed in a lab accident with unknown variables, it has never been duplicated. But there seems to be no force that can break it, dent it, or even scratch it.