A heart of gold

Someone with a heart of gold is a genuinely kind and caring person. They would do anything for you.

Examples:

My Mum has a heart of gold. She always thinks of others before herself.

John offered to do all the decorating this weekend; he's got a heart of gold.

If someone has a heart of stone they are the opposite of someone with a heart of gold; they are unkind and cruel. 

Example:

You'll get no sympathy from the boss. He has a heart of stone.