Areola, from Dictionary.com:
[uh-ree-uh-luh]
–noun, plural -lae
1. a ring of color, as around the human nipple.
2. a small interstice, as between the fibers of connective tissue.
Aureole, from Dictionary.com:
[awr-ee-ohl]
–noun
1. a radiance surrounding the head or the whole figure in the representation of a sacred personage.
2. any encircling ring of light or color; halo.
3. Astronomy . corona ( def. 3 ) .
4. Geology . a zone of altered country rock around an igneous intrusion.