Carat is a term used to describe the mass of gems and pearls, and also the degree of purity of precious metals. Carat of gemology (mass) unit of mass used, fundamentally, to weigh gems and pearls, and abbreviated ct. In this sense, one carat historically represented one hundred and forty (1/140) part of an ounce (205 milligrams). Since 1907, with the implementation of the decimal metric system, the decimal carat is used, which represents a weight of 200 milligrams (0.2 grams), therefore 5 carats is equivalent to 1 gram. Gold-plated carat (purity) Designates the purity of the gold used in jewelry. In this sense, a carat (abbreviated K or kt) represents a twenty-four (1/24) part of the total mass of the alloy that makes up the metal (approximately 4.167%). For example, if a jewel made with gold is 18 carats, its alloy is made of 18/24 (or 3/4) parts of gold and has a purity of 75%, while a piece of 24 carats is made of 24 / 24 parts of gold and therefore it is pure gold. Origin of the carat mass The term comes from the ancient Greek word keration (κεράτιον), which means carob, because the seeds of this fruit were used in ancient times to weigh gems and jewels due to the notoriously uniform size and weight of the seeds. When the Arabs adopted this unit of mass the name was deformed to quirat and it was deformed by carat when jumping into Spanish. Transition to carat purity In the year 309 d.C. the Roman Emperor Constantine I ordered to coin the first solidus, a 24 carat (mass) gold coin (about 4.5 grams), integrated into the Roman duodecimal system of weights and measures like 1/72 of a pound, each carat being siliquoa 1/1728 of a pound The solidus (from which the Castilian word salary comes), and its Arabic equivalent dinar (from the Latin denario, from which money comes), were the reference of the weight of gold until at least the 12th century, and its purity became the law of gold, that is, how many carat-mass (of the total 24 of the coin) are pure gold: 24 carat means 100% purity, 18 carat means that the alloy contains 75% , etc.
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