Identify the underlined phrase as an example of a simile, metaphor, or personification.
She had a heart of stone.
The underlined phrase, "She had a heart of stone," is an example of a metaphor.
Complete the following task based on the instruction and optional input provided.### Instruction:
Identify the underlined phrase as an example of a simile, metaphor, or personification.

### Input:
She had a heart of stone.

### Output:
The underlined phrase, "She had a heart of stone," is an example of a metaphor.