I was just curious if stage progression is based on score, time or some other factor since I seem to have unlocked new stages even after dying. I looked but couldn't find a manual or readme file to answer this question.

Okay, I know this is way way late, but for others who might have the same question.

Stages unlock based on jewels picked up in the previous level. At least the main 6 work like that. Never figured out how the unlock of the two extras work.