All gold to be worked into jewelry must be alloyed (mixed) with other metals to obtain sufficient strength. Gold is available in several different carat contents that indicate how much gold the gold alloy contains. 24 carat is 100% pure gold, 18 carat is 75% gold.The most common 18K gold alloy in Swedish trade, "Swedish red gold", has a gold content of 75%, silver 8-9% and copper 16-17%.
Gold can be discolored and it depends on chemical and mechanical phenomena, among other things it is affected by sulphides and chlorides in skin secretions, sweat, saliva, etc. It is the proportion of copper and how your body affects copper that causes the jewelry to be discolored.
In Sweden, all white and red gold produced is nickel-free, while it often contains nickel abroad. 18k gold works very well as piercing jewelry.