I think it would be more fun if there was a visual difference between real and fake gems, or some hint that the player could get to find which ones are real. That and the lack of sound are probably my two biggest complaints.
That said, I have to compliment your level design. It seems like you know what good levels look like, and how to reward players for putting in effort (putting rewards at the end of challenging platforming puzzles and in hidden corners, for example). I think this could be a lot of fun if you flesh it out. If you decide to do that, let me know - I'd love to play it.
Great work!