I thought the combat was a bit slow. I would have liked a version of it without the text after I understood how to do it.
I wish after selecting one gem, I would be able to hit B to unselect it.
Great art style. And I liked the turn based strategy and the general vibe of what the texts say. I don't think it needs an overworld.
Maybe increase the damage-to-healing ratio so the battles don't go on so long. And add some sort of tutorial so that your game teaches people instead of the description