ˌdiamond in the ˈrough noun [countable]
American English someone who behaves in a slightly rude way, but is really kind and generous Synonym : rough diamond British English

[TahlilGaran] Dictionary of Contemporary English ▲

diamond in the rough
someone or something whose good qualities are hidden.
This film is one of those diamonds in the rough, a wonderful gem that almost no one has noticed.

Etymology: based on the idea that you cannot see the beauty of a diamond (= jewel) when it is rough (= not yet cut and filled with brightness)

[TahlilGaran] English Idioms Dictionary ▲

diamond in the rough
a good person or thing that is hidden by a rough exterior
The man is a diamond in the rough and a very gentle person under his harsh exterior.

[TahlilGaran] English Idioms Dictionary ▲

diamond in the rough
n. phr. A very smart person without a formal education who may have untutored manners.
Jack never went to school but he is extremely talented; he is a veritable diamond in the rough.

[TahlilGaran] English Idioms Dictionary ▲