Hailey Bieber Gifts Justin Diamond-Studded Necklace Days After Her Birthday

Hailey Bieber is very quick to return a favor, because just days after she turned 23 ... the missus got her hubby something arguably nicer than the birthday gift he gave her.

Justin's showing off this new iced-out necklace he just got from L.A. jeweler Jadelle Beverly Hills. Even in the dimly lit pic, you can see just how damn blingy the chain is -- and we've learned Hailey actually picked it up for her groom.

The timing is interesting because Hailey's birthday was just 5 days ago, and the Biebs got his wife a glimmering diamond watch ... which we're told set him back $70k.

Our sources say Hailey turned around and one-upped her man on Tuesday with the chain -- which is similar to a traditional Cuban link, but Justin's is thinner and lighter.

As for the specs ... white gold covered in VVS super white diamonds, and valued at $90k. For the record, it's nowhere near Justin's bday.

Our sources say Hailey reached out to Jadelle -- a favorite shopping spot for the couple -- and surprised Justin with it in Miami, where they're currently on a working vacay. As you can see, Hailey's the one working..