Disruptions in Tooth Formation

Dental hypoplasia is the banding or pitting of the enamel due to metabolic stress when the tooth is being formed. Specific factors include inherited and infectious disease, changes in dietary habits, malnutrition and intoxication.
(Palubeckaite et al. 2002)

Courtesy: www.uiowa.edu/~c090247/ENAMEL%20HYPOPLASIA.htm

The antibiotic Tetracycline, when given to children, stains the enamel being laid down when the medicine is taken. It is no longer given to children. These types of stains are seen mainly in older adults today. This person took the drug all through her childhood. Usually there is a single stripe in the enamel corresponding to a single dose. Colors range from tan to grey.
Courtesy (picture and info): www.forsyth.org/oralpathology/case_018_answer.htm