Rings normal. Two causes of white rings on teeth are decalcifications and hypoplasia. Decalcifications can be associated with not brushing plaque off the surface or diet issues, so minerals have pulled out of enamel. Enamel hypoplasia is a defect in the pattern of tooth surface giving a color difference. This occured during it development as a child. Many reasons.
Dentist can help. A person who sees whitish marks on his teeth should ask his dentist about them during his 2-3 times a year check-ups and cleanings. Marks on the enamel might be from unevenness of the enamel surface, from staining, from plaque, from brushing too hard in one direction, etc... Dentists have lots of experience with the various enamel changes seen on teeth.