Have a heart of gold
Definition 1 of 1
Pro Tip 1/3Positive Connotation
Using 'have a heart of gold' suggests admiration for someone's character, emphasizing their intrinsic goodness. 
My grandmother would always help the less fortunate; she truly had a heart of gold.
Pro Tip 2/3Emotional Weight
The phrase carries emotional weight, making it suitable for heartfelt or sincere moments rather than everyday conversation. 
At the farewell, the principal said Mrs. Thompson has a heart of gold, which brought tears to our eyes.
Pro Tip 3/3Common Misuse
Avoid using the phrase sarcastically or in a situation where the person's actions are self-serving rather than altruistic. 
He donated not to show off but because he has a heart of gold.