Other forms: diamonds in the rough
A great artist with bad manners might be described as a diamond in the rough. The phrase usually refers to people with good qualities who have difficulty fitting into the environment where those qualities would shine most.
When diamonds are mined from the earth, they don't look like the sparkly gemstones we're used to seeing in earrings and necklaces. They're lumpy pebbles with uneven edges until they've been ground down and polished with special tools, and this process gives them the facets that help them reflect light so beautifully. Similarly, a person who is a diamond in the rough has a lot of potential or valuable skills, but something about their appearance or demeanor makes those qualities less apparent.