Have a heart of stone
Definition 1 of 1
Pro Tip 1/3Emotional Weight
The phrase carries negative emotions and indicates harshness in someone's personality. 
He dismissed their pleas without remorse; he must have a heart of stone.
Pro Tip 2/3Metaphorical Use
It's not literal — nobody has an actual stone heart. It's a metaphor for lack of empathy. 
I can't believe she just laughed at his misfortune. She has a heart of stone.
Pro Tip 3/3Character Assessment
Used to judge character, often indicating surprise or disapproval at one's cold behavior. 
Even though he's rich, he never donates to charity. He has a heart of stone.