heart of gold
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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.