A heart of Gold

Where did the phrase "A Heart of Gold" come from and what does it mean. I am doing a report and this is my subjuct. I can't figure out where this phrase came from. I need help!! please help me if you can. Thank you very much. Have a good day.

[Tdol]

There doesn't seem to be much about it. the American heritage says it comes from the 1500s:
A very kind and good nature, as in Bill is very generous; he has a heart of gold. This expression alludes to gold in the sense of "something valued for its goodness." [Late 1500s]


Source: The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company.
http://dictionary.reference.com/sear...rt%20of%20gold