Quite neat! For some reason it reminded me of "Riff: Everyday Shooter" - perhaps a combination of the music and the player/enemy designs?
The controls felt a little sluggish at first, but they were fine once I got used to them. The enemies were generally easy to avoid (even on hard mode), but the fact that one hit ends the game made it feel a little punishing? More seriously, gems would occasionally spawn inside walls; I think I've only seen that happen when a level starts, but I'm not sure.
But overall I had fun playing; nice entry!