Metallic Caster (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least 5 Earth-based Evocation spells.
You've refined your attunement to the earth so well that you've begun to realize that some metals have earth inside of them.
You can affect iron and other impure metals (Metals containing traces of earthen material) with your spells, treating them like metal or earth as the spell requires. For example, this might mean you can use Move Earth on iron to make it difficult terrain. Pure metals, like Platinum, ignore this feat. Additionally, earth up to as hard as iron can be moved by spells that normally only allow loose soil to be moved, including but not limited to Move Earth, and this affects such metals as well. Finally, if you are attacked by a weapon that uses impure metal or earth, you can use your reaction to force the attacker to succeed on a Strength saving throw against your spell save DC or miss the attack. This is before they roll, and if they succeed by 2 or less then they have disadvantage on the attack.
Warning: If you don't have 5 earth Evocation spells memorized for any reason, you can no longer use this feat until you meet it's requirements again. You still have the feat, it simply doesn't work.