18K (18 Karat) gold is an alloy of gold and other metals used in fine jewelry manufacture. 18 karat gold jewelry may be stamped 18K or may also be stamped .750 in the metal. The karat gold marking on a piece of jewelry represents the percentage of fine or pure 24 karat gold contained in the particular piece. In the case of gold jewelry marked 18K, the mark indicates the item’s composition is 18/24ths fine gold or 75% fine gold and 25% other metals.
Gold in its pure state (24K) is soft and malleable. Though beautiful in color, 24K gold is not a practical choice for everyday jewelry. But 18K gold is hard and strong. That is because karat gold is gold which has been alloyed with other metals to add strength, durability and even color. 18K gold, is composed of 75% fine gold alloyed with 25% other metals like silver, copper, nickel and palladium. The other metals used – and their proportions – impact the color of the alloy significantly. For example, yellow 18K gold can be bleached white by adding palladium, silver or nickel to the alloy, or turned rose colored by adding copper.