Metal Reactions

My skin is green
Copper reacts to the acidity in some people and turns skin green. The piece you're wearing might not be pure copper, but it might have enough copper in it to cause a reaction, and some people seem to have body chemistry that's prone to staining.

Sterling silver is 7.5 percent copper, but the most common stains from sterling silver jewelry are black, and occur when the metal tarnishes (darkens due to a reaction with gases in the air).

Has your silver jewelry ever left greenish or black marks on your skin? It's happened to most of us at least once or twice, and even though some people are allergic to certain materials, most staining isn't caused by an allergy--it's a reaction between our skin and the metals used in jewelry, especially when the jewelry touches areas that perspire.

Here is an article that explains the allergy
http://jewelry.about.com/od/jewelrymetals/a/skin_reactions.htm