The Short Answer
Copper is a metal that’s used for a lot of costume jewelries. Even though it’s already made into a piece of jewelry, the copper reacts with either the product on your skin or your skin itself in a common chemical reaction; leaving a greenish tint on your skin.
The Long Answer
Copper is an essential mineral, meaning it occurs naturally in the human body and nature. Copper has a signature reddish brown colour and is a malleable and soft natural metal.
In its purest form, copper is tough but not strong which is why why copper gets mixed with other metals to strengthen it. These are called copper alloy with the most common being brass and bronze.
Copper alloy is extremely durable and can hold up well over time with daily wear. However over time, copper oxidizes and a chemical reaction turns the metal green.
Therefore the oxidization sometimes leaves a greenish hue on your skin. This is not dangerous or permanent and can be washed off with soap water.
Alternatively, you can choose jewelry made from different metal like stainless steel, platinum, gold, silver and rhodium-plated.