* Base Metal Corrosion: Gold plating is a thin layer of gold over a base metal. The base metal is often copper, brass, or nickel, which can react with moisture, sweat, and body oils. These reactions cause the base metal to corrode.
* Copper Oxidation: When copper corrodes, it forms copper oxides, which have a greenish color. These oxides can transfer onto your skin, causing the green mark.
* Thin Gold Layer: The gold plating is very thin and can wear away over time, exposing the base metal beneath. This makes the green staining more likely as the jewelry ages.
* Individual Body Chemistry: The likelihood of staining also depends on your body chemistry. Some people have more acidic sweat, which can accelerate the corrosion process.
So, while gold itself is inert and won't react with your skin, the base metals commonly used in gold-plated jewelry are prone to corrosion and can cause a green discoloration.