I think what you are referring to is the interproximal areas between teeth
the lighter outer area is the enamel (lighter because it is more dense) and
the darker inside is the dentin (less dense) and the dark middle (nerve and
blood vessels (even less dense)  decay appears as darker spots in the enamel
and dentin  reminerilization can only occur in enamel, once it reaches the
dentin all bets are off get out the drill!

I think this is what you were talking about.

Your friendly neighborhood paleo dentist  :)

Alex