Mariah Carey -- Diamonds Get Her Every Time ... Fan Sucks Up with Jewelry

Mariah Carey is smiling from earhole to earhole because one of her fans -- with money to burn, apparently -- laid a cool 4 Gs worth o' jewels on the diva as a pretty sweet thank you gift.

Sources tell TMZ ... Mimi invited jeweler Eric Meier to her opening Vegas show at Caesars Palace, and to prove he's her biggest fan -- he handed her a pair of black diamond drop earrings in an 18 ct. rose gold setting.

We're told the Orion earrings are worth nearly $4k, but we're guessing he got a decent discount since they're available at his store, Diamonds Don't Lie. Still, it's the thought that counts ... as you can tell from Mariah's face.