Why do sterling silver earrings turn black?

Silver, especially low-grade ones, often tarnish when exposed to minerals and air. It leads to the process of oxidation and hence, may turn your silver, Black.
Recent studies have shown that tarnish develops when exposed to air and chemicals. A great way to keep seldom worn jewelry clean is to keep these in an airtight container or ziplock bag. You can wrap it in a soft cloth for extremely fragile jewelry and avoid chances of oxidation and tarnish.