a rough diamond

a rough diamond or a diamond in the rough

Meaning:
– a person of exceptional character
– a person with great potential but lacking polish and refinement
– a person who does not seem very polite or well educated at first, although they have a good character
– a person who has good qualities despite a rough exterior
– someone or something whose good qualities are hidden
– a person who is kinder and more amusing than they seem to be from their appearance and behavior

Examples:
1. Bob is intelligent and trustworthy but lacks sophistication, he is a rough diamond.
2. Mitchell may have been a rough diamond, but he was absolutely loyal to his employer.
3. Rickey looks a little messy, but he's a diamond in the rough.
4. She's a diamond in the rough – a little hard to take at times, but very elegant and cooperative.
5. This show is one of those diamonds in the rough, a wonderful gem that almost no one has noticed.
6. Her singing voice is beautiful, but she needs help with her gestures; she's a rough diamond.
7. Jack is intelligent and loyal but lacks manners – he's a rough diamond.

Origin:
This idiomatic expression is obviously a metaphor for the original unpolished state of diamond gemstones. It comes from the fact that when diamonds are newly mined – that is, before they have been cut and polished – they don't shine, in fact they look quite a lot like pebbles and are easily overlooked in their “rough” state.

From this comes the idea that a person can also be like a diamond in the rough or, in the more common idiom, “a rough diamond.” This means a person who has rough, uncultivated or even impolite manners, but at heart is a very good person with excellent qualities. It is more commonly expressed in the form ‘rough diamond'. The first recorded use in print is in John Fletcher's ‘A Wife for a Month, 1624' – “She is very honest, and will be as hard to cut as a rough diamond.”