Mineral deposits, hard water, bacteria, and mold all create rings around toilet bowls. The top of the bowl is constantly alternating between wet and dry conditions, promoting deposits of these contaminants and the formation of rings.
Mineral deposits and hard water cause pale brown stains. Black, orange and green rings are typically created by mold. Pink rings indicate the presence of Serratia marcescens, a common bacterium. Mold or bacterial rings can be treated by cleaning the ring with bleach or white vinegar. Stains from mineral deposits or hard water require the use of a non-bleach alternative, because bleach can set the stain.