"to cut your teeth" on something means to learn from it, usually when you are young or just starting out at something. “When I started to learn Spanish I cut my teeth on Mexican movies so I know a lot of slang.”
The lyrics ("Royals" by Lorde) are:
I've never seen a diamond in the flesh
I cut my teeth on wedding rings in the movies
She has never seen a diamond in real life ("in the flesh"), she only knows about them from movies. ie. she is not rich. It's true this is a strange way to say it but it is a song and you can say anything you want in songs. ;-)