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

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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.