IIRC there's always two crystals spawned somewhere (except immediately when you collect one - it has some short delay until it's respawned). Also, there's only a handful of places where they can be, but multiple good "vista routes" you might take to inspect where the gem is.
I agree with commenters that the game is too hard. Maaaybe it'd be ok if there were earlier levels that ramped up to that difficulty.
Thanks!