It's happened to the best of us - you find the perfect ring, make the purchase, and wear it non-stop until you notice a faint green line around your finger. While the common assumption is to write off the piece for its poor quality, there's actually a scientific explanation for this phenomenon.
Copper is one of the most popular culprits, as the metal tends to corrode when put into contact with anything from sweat to hand lotion. This leads to the creation of copper salts which then ultimately cause the faint green mark in question.
Sterling silver can produce a similar effect. Many gold vermeil and gold plated rings have a sterling silver base metal. Rather than a faint green mark, the oxidation of silver when put in contact with skin can lead to a more dark green or even black ring around your finger.
If a green hue on your finger is accompanied by itching, it's possible that you possess a specific metal allergy. Most metal allergies are easily treated, and can be avoided altogether by opting for different metal options.
The green skin is a sign of how your own body chemistry reacts to various metals, and has little to nothing to do with the quality of the jewelry. Always make yourself aware of the metal content of the piece, this should always be disclosed when buying jewelry!
To lessen the likelihood of your skin turning green, you can line the inside of your ring with clear nail polish. The most surefire way to avoid the green is to invest in solid gold pieces. Regardless, fashion jewelry should always be removed when coming into contact with moisture (swimming, washing your hands, exercising, cleaning, etc.)