to increase the hardness of gold for jewelry, gold is often alloyed with one or more other metals
gold is often used as a coating over less expensive metals. when a layer of 10k or 12k gold is permanently fused onto an ingot of less expensive metal, then shaped into a sheet or wire, the result is gold fill
gold plating is similar to gold fill, but the amount of gold applied can be lower
vermeil is gold-plated sterling silver
the word "karat" is reserved as a measure of purity of gold
the word "carat" is reserved for gem weight