Why does jewelry tarnish? Well, in one simple word--sulfur. Sulfur in the air interacts with metal to creating that dingy, and ultimately black, layer on your jewelry and the copper constituent is usually the culprit.
Copper, you say? Copper reacts with sulfur to create tarnish in the form of copper sulfides. Ever wonder what the stamp 925 on your silver jewelry actually means? Well, it designates it as sterling silver, but also the composition.