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What is a Carat?
A carat is a unit of measurement used to express the weight of precious stones. The word comes from the Greek name for the carob or locust tree. The dried seed of this tree was originally used by gem traders as the unit for weighing their gems. Because the seeds are not perfectly uniform, variations in the value of the carat developed in different areas. An international or metric carat of 0.2 grams (1 gram equals 0.035 ounces) was proposed in Paris in 1907. This value was adopted by the United States in 1913, and it is now generally accepted in most countries. One carat is equal to about 3.086 grains in the troy system.
The word "carat" is also used to express the purity of gold; in this sense, it is usually spelled karat. A karat is not a unit of weight but instead means 1/24th part. Thus, 24-karat gold is pure gold, whereas 18-karat gold contains 18 parts gold and 6 parts alloy.