This is really impressive, a very unique concept! The sprites are well made, and the fact that you can hold to jump higher adds a lot of depth, with the most valuable gems being higher and nearer the death area. The sound fits well, although I noticed that the jump sound is played every time you hit the spacebar, even when you are already jumping.
My only critique would be that it's too easy to just stay walk across the top of the screen without much jumping as you avoid all the obstacles. I know this means you won't get as many points, but I'd argue that for a platformer reaching the end of the level should be a challenge in itself.
Fantastic job, could definitely see this on the marketplace with a bit of polish!