When gold jewelry causes discoloration of skin, it is due to a chemical reaction between the jewelry and oils on the skin. Most gold jewelry is not actually solid gold and can contain different metals, such as copper or nickel, that react with the skin's natural acidity. The reaction causes the other metals to oxidize, and leaves a discoloration of black or green on the skin.
While pure gold does not react with acidity, other metals, such as copper and nickel, can be added to jewelry to change the color or to make it harder and more durable. When skin that is in contact with this jewelry is sweaty or is covered in lotion or another product, the other metals in the jewelry undergo a chemical reaction, and the oxidation leaves the skin with a dark green or black stain.