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.