When jewellery made of copper, brass, or bronze comes into contact with the skin, it tends to blacken or otherwise oxidise.
This occurs because the oxides produced by our bodies through sweat react immediately when they come into contact with these metals, turning the metal's surface as well as the point of contact black.
Copper, as I discuss in greater detail in the article [Copper Properties], has beneficial properties for the human body, as opposed to brass and bronze, which should be avoided by applying a layer of varnish to the internal descent of the rings to avoid tanning our fingers.