What do we mean when we say it today?
Any Neil Young fans out there know that "Heart of Gold" is also a song title. But it's much more than a song. It's become such a popular phrase that you'd be hard-pressed not to hear it around now and again.
People with hearts of gold might be rare, but this phrase sure isn't. We hear it all the time when someone is talking about how totally generous and earnest someone is.