I can only speak from my experience, but the older, darker stones that I passed (and they came after my 2nd flush) all floated. As I recall, they kinda freaked me out seeing them floating, but after asking questions on this website and reading, I knew they were better out than in!
Paulette G.
P.S. (I never knew Alolph Hitler was married. I thought he just had that girlfriend! Blows my mind)