If a person has a natural high acidity level in his/her skin, it is possible for his/her finger to "turn green" from silver or gold jewelry, or for silver jewelry to blacken especially when the weather is hot and humid. The green is actually caused by oxidation, and all silver will oxidize (it's also what gives silver a tarnished look). There are some costume jewelry items that cause the same effect. Most of them contain nickel. It's not poisonous in small quantities but it can provoke a reaction in people allergic to nickel. This "green finger effect" and the tarnishing of silver may also be an allergic reaction to alloy elements in sterling silver or gold. In some cases, the alloys, used to make jewelry more durable, may react with your body chemistry, chemicals in the air and household chemicals such as soap, salt and sun tan lotion causing a harmless green or black discoloration on the skin, which can be easily removed with soap and water, silver cleaner or with a polish cloth. For this reason, most retail silver jewelry and white gold jewelry is plated with rhodium, which serves as a defensive layer and prevents this reaction from happening to anyone with allergy.