carat karat : Jewelry Store, News, and Information

A carat is a unit of weight for diamonds and other gemstones. One carat equals 200 milligrams or 0.2 grams. Carat refers to the gemstones mass. When used with gold, carat refers to the purity of gold and not its weight. In the United States and Canada the spelling karat is usually used for the measure of purity, while carat refers to the measure of mass. Each karat represents 1/24th of the whole. 24-karat gold is pure gold. 12-karat gold is 50% purity. 18-karat gold is 75% purity. Jewelry of 18 karat gold would mean that 18 parts are pure gold, and the remaining six parts are other metals. To determine the percentage of gold, just divide the karat purity by 24 and multiply by 100. 10-karat gold is 41.67% gold = (10/24)100.

How did carat come to be used for both the weight of gemstones and the purity of gold?

Back in medieval times there was a gold coin called a mark that weighed 24 carats. In this case carat did represent the weight of the coin. One carat was equal to the weight of a coral tree seed. Pure gold was too soft to produce marks, so copper and other metals were added to produce a hard alloy. The purity of the gold in the coin was expressed in the number of carats of gold in the coin. As long as the coin weighed 24 carats, the measure of weight and purity of gold in the coin were equal. When this measure of purity was used for coins and jewelry of different weights, karat came to represent the purity of gold and not its weight.