14k Gold Vermeil with Red Cubic Zirconia
With gold vermeil jewelry (sterling silver with gold plate) it's the silver that's causing the reaction. Sterling silver isn't 100% silver, it's 92.5% silver, and 7.5% alloys. The other alloys, primarily copper, are responsible for the tarnishing reaction. Tarnish can be described as a chemical reaction that happens when silver is exposed to oxygen and sulfur.
Tarnish will accelerate when exposed to water, in high humidity areas, and with people who have a high Ph in their skin. Skin lotions, sunscreen, perfume, and hairspray can all also accelerate tarnishing.