Bella Thorne and Boyfriend Go Ring Shopping for Her Movie Premiere

Bella Thorne and her musician boyfriend went ring shopping together ... but it's not what you think.

Bella and Mod Sun -- who've been super close lately -- hit up Pristine Jewelers in NYC Thursday to get her a custom ring for the premiere of her new movie with Patrick Schwarzenegger, "Midnight Sun."

We're told the owners, Avi and Ofir -- celeb jewelers to the stars -- helped her pick out a ring with 7.5 karats in yellow diamonds ... surrounding a huge 15-karat emerald.

Bella's BF didn't leave empty-handed either -- he ended up walking out with a custom heart pendant with 22 karats in colorful stones.