@JeremyECrawford Player just got back to me. "Then why does a Rust Monsters attack dmg Adamantine per it's description? if it's magical it should be omitted from the substances a rust monster dmgs."
— Kantrism (@Kantrism) November 8, 2017
Adamantine is a metal. The object called adamantine armor in the magic item section of the Dungeon Master's Guide is a magic item that incorporates that metal. #DnD https://t.co/3n9ttt7YyW
— Jeremy Crawford (@JeremyECrawford) November 8, 2017
The object called +1 Weapon in the magic item section of the Dungeon Master’s Guide is a magic item that incorporates metal.
Makes no sense
JC was/is drawing attention to the fact that, against some players’ misperceptions, adamantine (and mithral, as a side note) isn’t inherently magical. Certain magic items use adamantine in their creation (just as certain rare materials are often used in the creation of different magic items), but a sword that happens to be forged from adamantine isn’t itself inherently magical unless enchanted to be so.