Sean just turned the big 3-0 and he celebrated by treating himself to some bling from NYC-based Avianne & Co Jewelers, and it's gonna weigh heavy on his hand, for sure.

The centerpiece is a stunning 4-carat Colombian emerald that's surrounded by 15 carats worth of VS diamonds. The total cost is $120k, or basically ... a new G-Wagon on his finger.

We're told the ring was made in New York but it was delivered to Sean in L.A. before his birthday, which he celebrated over the weekend with Jay-Z, Beyonce, John Mayer and, of course, Jhene.