Imagine someone creates a piece of art, but nobody notices it. Like a great book, which nobody reads because nobody knows that it exists.
What are some powerful words or metaphors for this condition?
Hidden gem comes to mind. The word gem is in itself used metaphorically:
something prized especially for great beauty or perfection
[M-W] 2 : a
Cf. also this earlier post.
People may also occasionally use a diamond in the rough; this phrase, which I originally suggested, appears to have a range of meanings, including "Someone who is basically good hearted but lacking social graces and respect for the law." (source), and therefore appears less suitable. Thanks to Oldbag for pointing out the ambiguity.