What is a carat and a karat?

★ wanayoo — archive 1999 http://howstuffworks.com/question64.htm

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Question

My fiancee gave me a 1 carat diamond in my engagement ring. What is a carat? And how does it relate to a karat, as in "18 karat gold"?

Answer

A carat is a unit of weight for diamonds and other gemstones. One carat equals 200 milligrams (0.200 grams). There are 453 grams in a pound (1,000 grams to a kilogram). Therefore, if your fiancee weighs 170 pounds, you have a 385,050 carat fiancee!

A karat, when used with gold, is a unit of purity. 24 carat gold is pure gold, but usually you mix gold with a metal like copper or silver to make jewelry (because pure gold is too soft). Each karat means 1/24th of the whole. So if a piece of jewelry is made of metal that is 18 parts gold and 6 parts copper, that is 18 karat gold.

Where did such a funny unit of purity come from? It turns out that a German gold coin called a mark was common about a thousand years ago. It weighed 24 carats (4.8 grams). The purity of the gold in the coin was expressed in the number of carats of gold present in this 24 carat coin.

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