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".
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:
- 999 (also known as three nines fine)
- 950 (the most common purity for platinum jewellery)
- 900 (also known as one nine fine)
- 999 (Fine gold equivalent to 24 carat, also known as three nines fine)
- 990 (equivalent to 23 carat; also known as two nines fine)
- 916 (equivalent to 22 carat)
- 833 (equivalent to 20 carat)
- 750 (equivalent to 18 carat)
- 625 (equivalent to 15 carat)
- 585 (equivalent to 14 carat)
- 417 (equivalent to 10 carat)
- 375 (equivalent to 9 carat)
- 333 (equivalent to 8 carat; minimum standard for gold in Germany after 1884)
- 999 (Fine silver used in bullion bars, also known as three nines fine)
- 980 (common standard used in Mexico ca.1930 - 1945)
- 958 (equivalent to Britannia silver)
- 950 (equivalent to "French 1st Standard")
- 925 (equivalent to Sterling silver)
- 900 (equivalent to "Coin silver" in the USA, also known as one nine fine)
- 875 (could be found in former USSR)
- 830 (common standard used in older Scandinavian silver)
- 800 (minimum standard for silver in Germany after 1884; Egyptian silver)