Here's why:
* Idiom: An idiom is a phrase that has a figurative meaning that is different from the literal meaning of the words. In this case, "heart of stone" doesn't literally mean someone has a heart made of stone. It means they are lacking in compassion or empathy.
* Personification: Personification is giving human qualities to inanimate objects or abstract concepts. While "heart of stone" implies a lack of emotion, it doesn't literally give the heart human qualities.
Let me know if you'd like to explore other examples of idioms or personification!