Your showing off that new ring your boyfriend gave you last night at dinner. That's not real, REAL gold doesn't turn your finger colors. To your horrified eyes should appear a black ring of discoloration on your hand. Well your friend couldn't be more wrong. The most common reason for discoloration is metallic abrasion, caused by makeup.
Cosmetics often contain compounds harder than the jewelry itself.
They wear off and rub off tiny particles. Very finely divided metals always appear black rather than metallic, it sticks, forming a black smudge. To prevent this, you could try switching,cosmetics. Another cause is actual corrosion of the metals. Gold itself does not corrode, but the primary alloys of silver or copper will do so under moist conditions.