Warmaster Horus
Explorer
But it's not. It's mundane armor made out of a different material.
One more reason I miss 3.PF...
Or not. It's the first item in the Magic Items A-Z list in the DMG on p.150. The header to this list says that, "A magical item's description gives the item's name, its' category, its' rarity and its' magical properties."
Therefore Adamantine Armor is medium/heavy armor of an Uncommon rarity that has the magical property of making any critical hit against the wearer a normal hit.
You may think it's just a different metal, and perhaps there are non-magical adamantine armors out there we haven't heard of yet, but the one in the DMG is most certainly magical.