"Ruwe bolster blanke pit" aka a Rough Diamond

We have a proverb in Holland that says: "Ruwe bolster blanke pit". More or less this means don't only look at the outside, but also at the inside. I had the assumption that this proverb was related to the chesnut tree. This picture shows something different, or maybe it was open too long already.
I believe in English you can compare it to a "Rough diamond or Golden heart".