Just because you like wearing a ring doesn't mean it's made of high-quality material. Some costume jewellery and copper rings can leave green stains on your fingers. Prevent this by coating the inside with nail polish.
Photo by Iris
This creates a barrier between the ring and your finger, which prevents it from being able to stain your finger. The layer will rub off after a while, so if you wear a ring regularly, you may need to re-apply.