Pure silver(99.9%) is a precious metal along with sterling silver. Sterling silver is an alloy of silver containing 92.5% by weight of silver & 7.5% by weight of other metals, usually copper.

Because copper is a vital part of the composition of sterling silver(925 silver), it has a tendency to tarnish when exposed to salty air, chlorine, sulfur, humidity, perspiration, cosmetics, household bleach & other strong chemicals.

The exposure to these things result in the green tint you may see on the skin. Keep in mind the varying chemistry on one's skin, severity of sweating, products & oils used; causes the varying reactions. The green tint left on the skin is harmless. Just because your skin slightly turns green while wearing sterling silver, does not mean that the jewelry is fake or of a low quality. It's simply doing what sterling silver does. Sterling silver is still more durable than silver plating.