A: Who is that girl John hangs out a lot with now?

B: She is a new graduate from a film academy. She may seem shy and ordinary right now, but give her a couple of more years — she is a diamond in the rough.

Note: The phrase is metaphorical and relates to the fact that naturally occurring diamonds are quite ordinary at first glance, and that their true beauty as jewels is only realized through the cutting and polishing process. A diamond in the rough is someone (or something) that has hidden exceptional characteristics or future potential, but currently lacks the final touches that would make them stand out from the crowd.