In a lot of folklore, faries are hurt by or reppeled by iron
Concept:
Due to Bilbos' fae heritage, he's allergic to iron.
He would say it irritates his skin, but truthfully, it burns him. Even small brushes against the metal make it look like he was burned by a hot brand for days.
He obviously doesn't think to bring it up with anyone. It didn't seem important until Thorin's hand gently brushed the back of his, and Bilbo pulled back with a hiss.
Thorin stared wide-eyed (and a little heartbroken) for a second until he spotted the strip of red skin Bilbo was trying to hide.
"You're hurt, when did that happen?" Thorin went to take the hobbits hand for a better look, but Bilbo skittered back in a slight panic.
"Ah no it-It..well I think it was your ring," Bilbo said.
"My ring?"
Bilbo revealed the small rectangular burn on the back of his freckled hand and Thorin had to bite back a gasp.
"I'm allergic to iron you see, it ma-"
Before Bilbo could even finish the sentence, Thorin had wrenched his ring off his finger and thrown it with all his might into the forest.
Bilbo couldn't quite find his words for a few minutes as he stared slack jawed in the direction the ring had vanished into.
Thorin had just thrown off his ring. Thrown it. His ring he had worn every day Bilbo had known the dwarf. He just threw it into some bushes and would likely never see hide or hair of it again.
Because it had hurt him.
