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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 (i.e. '585', '750', etc.) rather than '14K', '18K', etc., which is used in the United Kingdom and United States.
It is an extension of the older carat (karat in North American spelling) system of denoting the purity of gold by fractions of 24, such as "18 carat" for an alloy with 75% (18 parts per 24) pure gold by mass.
The millesimal fineness is usually rounded to a three figure number, particularly where used as a hallmark, and the fineness may vary slightly from the traditional versions of purity.
The most common millesimal finenesses used for precious metals (and the most common terms associated with them):
- 999.5 What most dealers would buy as if 100% pure; the most common purity for platinum bullion coins and bars
- 999 (three nines fine)
- 950 The most common purity for platinum jewelry
- 900 (one nine fine)
- 999.9 (four nines fine) Ultra-fine silver used by Royal Canadian Mint in the Canadian Silver Maple Leaf
- 999 (Fine silver or three nines fine) Used in Good Delivery bullion bars
- 980 Common standard used in Mexico ca.1930 - 1945
- 958 E.g, Britannia silver
- 950 E.g, French 1st Standard
- 925 (Sterling silver)
- 900 (one nine fine or "90% silver") E.g., all 1892-1964 U.S. silver coins
- 835 A standard predominantly used in Germany after 1884
- 833 A common standard for continental silver especially among the Dutch, Swedish, and Germans
- 830 A common standard used in older Scandinavian silver
- 800 The minimum standard for silver in Germany after 1884; Egyptian silver; Canadian silver circulating coinage
- 750 An uncommon silver standard found in older German, Swiss and Austro-Hungarian silver
- 720 E.g., many Mexican silver coins