‘A diamond in the rough’
Meaning: A person of good character or potential, but lacking polish and refinement
Explanation: This phrase, also expressed as ‘rough diamond’, refers to the uncut and unpolished state of diamonds when they are mined. It first appears in 1624 in John Fletcher’s AWife for a Month: “She is very honest, and will be as hard to cut as a rough diamond.”
Example:He seems very abrupt and uncouth, but he’s a great guy – a diamond in the rough.