Here's the difference:
* Metaphor: A figure of speech that directly compares two unlike things without using "like" or "as".
* Simile: A figure of speech that compares two unlike things using "like" or "as".
In "heart of stone," the speaker is comparing someone's heart to a stone, implying they are cold and unfeeling. It's a direct comparison, not a comparison using "like" or "as".
Therefore, "heart of stone" is a metaphor, not a simile.