What is a metaphor that is kind of like "diamond in the rough?"?

I'm reviewing a dystopian novel with a teenager as the main character. It looks like the kind of novel you'd find next to Divergent and the Maze Runner, so I presumed at first that it would be a visceral, pandering, fun-but-substanceless read. I was dead wrong. This is a novel of great quality, and it had... show more I'm reviewing a dystopian novel with a teenager as the main character. It looks like the kind of novel you'd find next to Divergent and the Maze Runner, so I presumed at first that it would be a visceral, pandering, fun-but-substanceless read. I was dead wrong. This is a novel of great quality, and it had more in common with the likes of Ender's Game. I was going to describe it as a diamond in the rough because it is a thing of great quality that I found among things of lesser quality. Then I looked up what that metaphor actually meant. It means a thing of quality that's rough around the edges or in need of polishing. That's not what I want to express. Do you guys have a better metaphor I could use?
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