diamond in the rough

а diamond in the rough (someone or something with potential or talent but lacking training or polish) — нешлифованный / неограненный алмаз; неотесанный

Example 1: It once changed the course of a young man's life. A young man who liked this lamp was more than what he seemed. A diamond in the rough. (Aladdin)

Example 2: She's a diamond in the rough. — Талант у нее есть, но над ним еще нужно поработать

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.