Also, a rough diamond. A person of exceptional character or potential but lacking polish and refinement. For example, Jack is intelligent and trustworthy but lacks manners—he's a diamond in the rough. [Early 1600s]

Someone or something with potential or talent but lacking training or polish: “Her singing voice is beautiful, but she needs help with her gestures; she's a diamond in the rough.” This phrase refers to the fact that diamonds found in nature are rough and uneven. They must be cut and polished to bring out their true beauty.