Gold in its purest form does not leave black or green marks on skin, but the other metals mixed in with gold jewelry, such as copper, nickel and silver, can cause discoloration on skin. Discoloration usually occurs when these metals interact with perspiring skin.
Nickel, copper and silver are often mixed with gold to create an alloy. The higher levels of these metals contained in a piece of gold jewelry, the lower the karat. Copper causes green discoloration on the skin while silver causes black marks. To avoid green or black marks on skin from gold jewelry, it's best to wear gold jewelry that contains 18-karat levels of gold or higher.