Book recommendations for a young kid?

My niece's 7th birthday is coming up and she loves rocks. (As in, will talk your ear off for over an hour about the rocks she found that day.) Her library has only one rock/gem book for kids that she has read cover to cover and I'd like to give her a few more.

Do any of you have recommendations? She reads around the 4th-5th grade reading level. Some of the books I've found are too... easy? She's been into rocks for 1.5ish years and has learned a lot about them already. I'm looking for a book that goes a little more in depth for a curious child that loves to read and learn.

Thanks!