rough diamond 
noun
1An uncut diamond.Russia will supply 30,000 carats of rough diamonds for jewellery and 1 million carats of technical diamonds....
- The domestic diamond industry may soon get rough diamonds directly from African countries.
- The venture is a first for the diamond industry, where the world of diamond mining, which produces rough diamonds, and the world of jewellery retailing have historically remained very far apart.
1.1British A person who is generally of good character but lacks manners, education, or style: my grandfather was a bit of a rough diamond...
- Exuding coarse manners, she's every inch a rough diamond.
- I think he is a good soul but he's a rough diamond.
- I do not think that the defendant would object if I describe him as a rough diamond.