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.
Now imagine what Eric would have given to be the guy she cuffed and blindfolded onstage.