The metaphor "Heart of Stone" is in reference to someone who has a very cruel and cold nature. If you are considered to have a heart of stone then you are not the type to demonstrate sympathy, but instead a stern personality. You could even be depicted as a villain for not even smiling, since you are so stone hearted. Having a reputation of being stone hearted would result in minimal friends and people surrounding you because you are not emitting the greatest of energy.