Green skin is copper — not nickel, and not an allergy. Here is which metals do it, why plating only delays it, and why "what is it plated over" is a better question than "is it gold."

A flat back stud is an earring with a smooth disc behind the lobe instead of a butterfly clutch. Here is the anatomy, the actual measurements, and why the back is what makes it an earring you can leave in.