In order to avoid confusion with “carat”, the unit of weight for precious stones, the German word karat, usually abbreviated to the letter “k”, has been adopted as a measurement of the fineness of gold. One karat equals 1/24th part of pure gold in an alloy: 24k gold is therefore pure gold.The word itself derives from the Greek keration, an ancient unit of currency that was worth one third of an obol (one eighteenth of a drachma). 18k gold comes in several colors, which can be the result of different alloys: • Yellow gold: 75% gold, 12.5% silver, 12.5% copper • Red gold: 75% gold, 25% copper • Pink gold: 75% gold, 21% copper, 4% silver • Green gold: 75% gold, 25% silver • Grey-blue gold and blue gold: contains an iron alloy • Purple gold: contains the same iron alloy plus silver, copper and aluminum • White gold: in jewelry, 75% gold and an alloy of palladium, copper, silver and iridium; in watchmaking, 75% gold and an alloy of palladium, copper and zinc. (The exact proportions of metals used to create the white gold alloy used by Cartier remain strictly confidential). Legal standards of fineness for gold in France were set on the nineteenth day of the second month of the Republican calendar in the year VI (November 9th, 1797) at 920/1000th (22k), 840/1000th (20k) and 750/1000th (18k).Adding 2/24th of another metal to 24k pure gold produces 22k gold, and so on. Note that adding another metal, usually silver and/or copper, will harden gold which, when pure, is too malleable.