Joe is not that bad, you know! Deep down, he’s really kind even though he seems to lack good manners at times. He’s a rough diamond (i.e. a person who is generally of good character but lacks manners, education, or style).
Joe n'est pas si mal, tu sais ! Au fond, il est vraiment gentil même s'il semble parfois manquer de bonnes manières. C'est un diamant brut.
Diamand brut does not sound natural at all to my ears. What are some idiomatic ways to convey this meaning of 'rough diamond'?