Alloying elements, or other metals, are added to gold jewellery to improve the toughness of the metal. In gold that is karated, a balance of metals in the non-gold percentage exists. It is these metals that provide the different colours and toughness of karated golds.
Some individuals falsely believe that it is faulty manufacturing or under-karating that makes a piece of gold jewellery turn itself and/or the skin it touches black. The most likely reason for this result is metallic abrasion caused by cosmetics and/or creams on the skin.
Cosmetics often possess compounds that are stronger than the gold itself, so they can wear off tiny pieces of the gold. Finely cut particles of metal look black rather than metallic. These particles create a dust that, when it touches absorbent surfaces such as skin, makes a black smudge or stain around the finger.
You may want to switch whatever cosmetics you are using and see if that makes a difference. You should also remove your ring before putting on lotion and then clean the small area with soap and water where the ring will come into contact with the skin.
Another cause of black smudging from your gold ring could be the corrosion of the metal itself. According to Tom Baker, a wastewater treatment analyst, gold itself doesn't corrode, but its main alloys (silver and copper) do. This can cause a dark chemical compound in moist conditions.
There is another cause of dark smudges. Baker states that when you sweat, fats and fatty acids are released and can result in the corrosion of 14-karat gold, particularly when exposed to warm temperatures and air. This problem is more pervasive in semitropical and coastal areas where chlorides mix with sweat to create a corrosive element change.
You might consider cleaning your rings with soap and water. Use a gentle soap with a toothbrush to really get in all the crevices. Not only will you help keep your fingers clean, but your jewellery will look much more brilliant.