Carat weight can be 10, 12, 14, 18, 22, or 24. The higher the number, the greater the purity. To be called “solid gold,” gold must have a minimum weight of 10 carats. “Pure gold” must have a carat weight of 24, (though there is still a small amount of copper in it). Pure gold is so soft that it can be moulded by hand.
The term “carat” comes from “carob seed,” which was the standard for weighing small quantities in the Middle East. It was believed that carob seeds had a uniform weight but studies have shown that these seeds display as many variations as any other seed.
The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.