@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