Justin Bieber and Selena Gomez can't quit each other -- but is it super serious? Some sources are saying that Biebs is so determined to make it work that he's looking at engagement rings for Sel!

Rumor has it that Bieber was discussing rings with celeb jewelry designer Neil Lane. "Justin is planning to go through final designs with Neil in the next week," a source told the Daily Star (via ContactMusic). "At the moment his favorite idea is a huge, canary yellow, princess-cut diamond surrounded by smaller diamonds and set in white or yellow gold."

The man's got good taste!

As for when he'd pop the question? Sources say he's looking at Gomez's 21st birthday this July.

A source added, "Justin kept relentlessly pursuing her with text messages, emails and phone calls, despite the fact that she dumped him last year. Justin's mom Pattie also put pressure on Selena after coming to the conclusion she was the only one who could keep him on the straight and narrow."

Of course, they're super rich and super young, so maybe the ring isn't an engagement ring -- after all, Bieber previously gave Gomez a ring that wasn't an engagement band, but rather just a gift. Considering Gomez denies even being involved with Bieber anymore, only time will tell!