I just learned the coolest thing today! I love working with copper, but I always worry about it turning my customer's skin black or green, and there's no way of knowing ahead of time if it will do it or not because everybody is different.
I know that copper is an essential element in nutrition, and I know that skin is a digestive organ, but I never put two and two together before. I just read that if copper turns your skin colors, it's because you're deficient in the mineral and your skin is eating it! The color it leaves behind is the oxidized leftovers. If the color is bothering you just keep washing wherever the metal touches you, and then put the jewelry back on. In a matter of weeks or months, you will no longer get a reaction from the metal, because your body will be "full" of all of the copper it needs, and won't be absorbing any more.
Wow! Who knew?