Copper within the sterling silver is the likely culprit when a piece of jewelry turns its wearer's skin green, according to About.com. The green color on the skin is not dangerous, but some people may have a rash or other reaction.
Sterling silver is generally composed of 92.5 percent pure silver, and the remaining 7.5 percent usually contains copper, according to SnailSilver. The presence of copper can cause green skin, although it may indicate that the jewelry is of lower quality than sterling silver. Silver-plated jewelry and lower-quality silver alloys have higher copper content, according to SnailSilver, so they are more likely to cause skin to turn green. Exposure to acids causes sterling silver jewelry to oxidize and tarnish, notes About.com, and this tarnish darkens the skin.