I really like the demo, the game feels, the music, and especially the pixel art are on point.
I like the onboarding and how it teaches the player how to play but maybe adding some indication on which button should be press will be a great addition and would avoid the player searching for the good button or combination of buttons to press. but maybe it's not needed and is part of the self-learning process.
After my first run, I was a bit lost on what to do, since there were no gems in the first level, but after some runs, I understand and start to try to beat the level getting the maximum of gems. I really like how the game difficulty is evolving and the level design is procedural and the fact that the player need to learn and understand the different chunk of it, the learning process is really satisfying.
overall it's a really good demo and I'm really hyped by the game. I think in the final version if the game helps a bit more the player to comprehend how everything works, it will be an amazing game.
keep the good work!