1
general::
Also, a rough diamond. A person of exceptional character or potential but lacking polish and refinement. For example, Jack is intelligent and trustworthy but lacks manners? he's a diamond in the rough. [Early 1600s]
American Heritage Idioms
2
general::
Phrase(s): a diamond in the rough
Fig. a person who has good qualities despite a rough exterior; a person with great potential. • Sam looks a little scruffy, but he’s a diamond in the rough. • He’s a diamond in the rough—a little hard to take at times, but very smart and helpful.
McGrawhill's American Idioms And Phrasal Verbs