James Packer's 18 karat yellow heart-shaped diamond gift to Mariah Carey

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

According to TMZ, 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.