One of the most common causes of gold smudges is the presence of makeup or cosmetics on the skin or clothing. Compounds in cosmetics which are harder than gold, will cause tiny particles of gold to be rubbed off the jewelry. The purer gold is, the greater problem. Changing from 14K to 18K does not help this problem. These particles will appear to be black when spread out onto your skin or clothing. To avoid this remove your jewelry when applying cosmetics and use soap and water to clean the areas of skin that will touch your jewelry.
Another cause of discoloration is body chemistry. Chemicals such as chlorides and sulfides contained in perspiration may react with metals contained in the gold alloy, causing a black mark on your skin. Eating lots of tomatoes or drinking may change from time to time. A ring, which previously caused no problem, could suddenly start to smudge your finger. Exposure to chemicals outside the body can also have the same effect. Salt reacts with the alloys in yellow gold to cause corrosion. Eating a box of salted popcorn in the movies may cause your jewelry to darken your skin.
Two 14K gold rings may have the same amount of pure gold but different alloys. One ring might discolor and the other may not.
Metallic Abrasion
Corrosion
The Tarnishing of Sterling Silver Jewelry