Mariah Carey -- My Billionaire BF Spent $500K to Make My Chest Sparkle

Mariah Carey’s boyfriend James Packer knows the way to her heart is putting half a mil worth of rocks on her chest.

Packer gave Mariah the hookup Wednesday night -- it's an 18 karat yellow heart-shaped diamond. We're told he gave to her right before she went on stage for her Vegas show at Caesars Palace -- which explains the shout out she gave him.

Carey thanked JP for "a new heart" ... right before she sang "Hero" that night.

Guess James is cool with people not looking Mariah in the eyes when they talk to her, 'cause ... damn!