Gold is naturally yellow and can be found in a number of different purities. The purest form of gold is 24-karat, but this is seldom seen in rings because it is too soft for setting a diamond. Most commonly, 14-karat and 18-karat gold are used. When we talk about the different "karats" of gold we are referring to the degree to which the gold is mixed with other metals. For example, an alloy (a mix of metals) of 14 parts gold and ten parts other metal would be 14-karat gold. The more "other metal" used in the process, the stronger the gold. The varied "colors" of gold are the result of specific alloys that have been developed to change gold from its natural color. Red gold, for example, is created when gold is blended with copper. Typically it is the metals used to create gold alloys that people are allergic to when they have an allergic reaction to gold jewelry. An allergy to gold itself is extremely rare. Revered by everyone from the ancient Egyptians to the ancient Greeks, gold has been one of the most popular materials for jewelry throughout history. The exploration of the New World by European explorers was frequently in search of new sources for gold, especially in South America where the Aztecs considered gold a product of the gods.