Actually I’ve seen people mention it since the first episode.
It’s Capaldi’s wedding ring. He never takes it off, even when acting.
To add to the story, he refuses to take it off because when his acting career was struggling, his wife never gave up on him. When he landed his first major gig, he decided to not take it off, to represent he was there because of his wife’s support.
This is the sweetest and best thing I’ve eve heard about any behind-the-scenes ever
Remember when he’s told the tradition is to give that chocolate box to someone “you love" and he immediately jumped off the stage and ran to his wife in Seoul without even a second thought?
this is the reason i love and RESPECT him this much… just… *sniffs*