A ring that leaves a green mark is not dirty and the wearer is not unusual. The reaction is copper meeting sweat, and both the metal and the conditions are predictable.
Copper is in almost every jewellery alloy
Pure gold and pure silver are too soft for jewellery, so they are alloyed. Copper is the most common hardening metal in both cases.
Costume jewellery frequently uses brass or bronze, which are copper alloys with a much higher copper content than precious metal alloys.
So the difference between a piece that marks skin and one that does not is largely a question of how much copper is exposed at the surface.
Sweat provides the reagents
Skin carries water, salts, fatty acids and a mildly acidic surface film. Together these are enough to slowly corrode copper at the contact point.
The reaction forms copper salts, which are green or blue-green, and those compounds transfer onto skin where the metal has been resting.
Higher humidity, heat and physical activity all increase sweat and accelerate the process, which is why the marks are seasonal for many people.
Personal chemistry genuinely varies
Skin pH, sweat composition and how much someone perspires differ substantially between individuals and change with diet, medication and activity.
Two people wearing identical rings can therefore have completely different experiences, and neither is doing anything wrong.
Hand creams, sunscreens and cleaning products leave residues that alter surface chemistry further, and these are a common trigger in someone who never had the problem before.
Plating delays it rather than preventing it
A plated piece separates the copper alloy from skin, and while the plating is intact no reaction occurs at the surface.
Once wear opens the plating at an edge or on the inside of a band, the exposed base is in direct contact and marking begins.
Rhodium plating over silver and a clear lacquer over brass both work on the same principle, and both fail at the points that receive the most friction.
What actually stops it
Metals that do not corrode in these conditions, including stainless steel, titanium, niobium and high-purity gold, do not produce the reaction at all.
Keeping pieces dry, removing them for exercise and washing, and wiping them after wear all reduce contact time with sweat.
The marks themselves are harmless and wash off, and they should not be confused with an allergic reaction, which is usually to nickel and presents as irritated skin rather than as a stain.