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.
We're told the 2 dropped over $100k total on their new bling ... which they happily flaunted at the premiere.