Interesting game! Here are my comments:
The Good:
> I'd say you nailed the arcade feeling of the game. It's something you wanna keep on trying again and again to increase the score. I managed to score 3,482 in my best run.
> The mechanic is pretty unique (though its adherence to the theme is a bit of a stretch... but nevertheless clever! wouldn't have thought of that)
> Because of the simplicity of the mechanic (only one button), I think it could work very well as a mobile game!
> Graphics are pretty awesome. I especially liked the "GAME OVER" animation.
Possible points of improvement:
> I think some sense of progression would be nice.
> The enemies that appear on the side should be telegraphed. Especially when there are gems that appear near the edge of the screen that are too risky to get because an enemy could suddenly spawn.
> Some sound would be nice as well, if you decide to continue working on it.
Overall good game! Thanks for this.
P.S. Please consider giving feedback to our game as well. We are in need of more ratings. https://itch.io/jam/gmtk-2018/rate/300343