Pure gold does not tarnish. However, due to its inherent softness, gold is often alloyed with other materials when used to make jewelry, and these alloys may tarnish with time. The amount of gold in the alloy is represented via karats. Twenty-four-karat gold is pure gold, with no additional metals added, while 18-karat gold is an alloy that is 75 percent gold. The lower you go in terms of karats, the less gold there is in the alloy. The less gold there is in the alloy, the more likely it is to eventually tarnish.