In precious metals, it is understood by fineness, the proportion by weight in which the pure precious metal is in an alloy. There are two common ways of expressing fineness: millesimal fineness expressed in units of parts per 1,000 and karats used for gold. Karats measure the parts per 24, so that 18 karat = 18⁄24 = 75% and 24 karat gold is considered 100% gold.
Millesimal fineness is a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy. For example, an alloy containing 75% gold is denoted as "750". Many European countries use decimal hallmark stamps rather than "14K", "18K", etc., which is used in the United Kingdom and the United States.
The karat (US spelling, symbol K or kt) or carat (UK spelling, symbol C ) is a fractional measure of purity for gold alloys in parts fine per 24 parts whole.
24-karat gold is the purest. As it is not combined with any other metal, it is very malleable and of a lesser consistency. 24-karat gold is not used in jewelry.
18-karat gold is composed of 18 parts of gold and 6 parts another metal (forming an alloy with 75% gold and 25% of another metal). 12-karat gold is composed of 12 parts gold and 12 parts another metal, and so forth.
However, this system of calculation gives only the mass of pure gold contained in an alloy. The quantity of gold by volume in a less-than-24-karat gold alloy differs according to the alloys used. For example, knowing that standard 18-karat yellow gold consists of 75% gold, 12.5% silver and the remaining 12.5% of copper (all by mass), the volume of pure gold in this alloy will be 60% since gold is much denser than the other metals used.
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