The fineness of a precious metal refers to the ratio by weight of the primary metal to any added base metals or impurities. Many precious metals are used in the form of alloys. Other metals are added to increase hardness, to make the metal more practical for use in such items as coins and jewelry, or to decrease the cost of the alloy. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5%, by mass, of other metals, usually copper.
Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000 and karats used only 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 (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 karat system of denoting the purity of gold by fractions of 24, such as "18 karat" 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)
- 100 (six nines fine) The purest gold ever produced. Refined by the Perth Mint in 1957.
- 999.99 (five nines fine) The purest type of gold currently produced; the Royal Canadian Mint regularly produces commemorative coins in this fineness
- 999.9 (four nines fine) E.g., ordinary Canadian Gold Maple Leaf and American Buffalo coins
- 999 (24 karat, also occasionally known as three nines fine) e.g., Chinese Panda coins
- 995 The minimum allowed in Good Delivery gold bars
- 990 (two nines fine)
- 986 (Ducat fineness) Formerly used by Venetian and Holy Roman Empire mints; still in use in Austria and Hungary
- 958.3 (23 karat)
- 916 (22 karat) Historically the most widely used fineness for gold bullion coins; currently used for British Sovereigns, South African Krugerrands and American Gold Eagles
- 900 (one nine fine) Mostly used in Latin Monetary Union mintage (e.g. French and Swiss "Napoleon coin" 20 francs)
- 834 (20 karat)
- 750 (18 karat)
- 625 (15 karat)
- 585 (14 karat)
- 417 (10 karat)
- 375 (9 karat)
- 333 (8 karat) Minimum standard for gold in Germany after 1884 
- 999.9 (four nines fine) Ultra-fine silver used by the Royal Canadian Mint for their Silver Maple Leaf and other silver coins
- 999 (Fine silver or three nines fine) Used in Good Delivery bullion bars and most current silver bullion coins
- 980 Common standard used in Mexico ca.1930 - 1945
- 958 E.g., Britannia silver
- 950 E.g., French 1st Standard
- 925 (Sterling silver)
- 917 A standard used for the minting of Indian silver(rupees), during the British raj
- 900 (one nine fine or "90% silver") E.g., all 1792-1964 U.S. silver coins
- 835 A standard predominantly used in Germany after 1884, and for the minting of coins in countries of the Latin monetary union
- 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
Karat purity is measured as 24 times the pure mass divided by the total mass:
- is the karat rating of the material,
- is the mass of pure gold in the alloy, and
- is the total mass of the material.
Therefore, 24-karat gold is pure (also includes down to 99.95% gold by mass), 18-karat gold is 18 parts gold, 6 parts another metal (forming an alloy with 75% gold), 12-karat gold is 12 parts gold (12 parts another metal), and so forth.
In England, the karat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of fineness (that is, 99.2% purity) could have been described as being 23-karat, 3-grain, 1-quart gold.
The karat system is increasingly being complemented or superseded by the millesimal fineness system, in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy; e.g. 18-karat gold, 75% Au, would be called 750.
- 58.33% - 62.50% = 14k (acclaimed 58.33%)
- 75.00% - 79.16% = 18k (acclaimed 75.00%)
- 91.66% - 95.83% = 22k (acclaimed 91.66%)
- 95.83% - 99.95% = 23k (acclaimed 95.83%)
- 99.95 and 100 = 24k (acclaimed 99.99%)
However, this system of calculation gives only the mass of pure gold contained in an alloy. The term 18-karat gold means that the alloy's mass consists of 75% of gold and 25% of alloy(s). The quantity of gold by volume in a less-than-24-karat gold alloy differs according to the alloy(s) 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: 19.32 g/cm3 for gold, 10.49 g/cm3 for silver and 8.96 g/cm3 for copper.
This formula gives the amount of gold in cubic centimeters or in milliliters in an alloy:
- is the volume of gold in cubic centimeters or in milliliters,
- is the total mass of the alloy in grams, and
- is the karat purity of the alloy.
To have the percentage of the volume of gold in an alloy, divide the volume of gold in cubic centimetres or in millilitres by the total volume of the alloy in cubic centimetres or in millilitres.
For 10-carat gold, the gold volume in the alloy represents about 26% of the total volume for standard yellow gold. Talking about purity according to mass could lead to some misunderstandings; for many people, purity means volume.
Karat is a variant of carat. First attested in English in the mid-15th century, the word carat came from Middle French carat, from Italian carato, both which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn") (diminutive of κέρας - keras, "horn"), also from Arabic qīrāṭ meaning "fruit of the carob tree," also "weight of 4 grains," (قيراط) and was a unit of mass though it was probably not used to measure gold in classical times.
In 309 CE, Roman Emperor Constantine I began to mint a new gold coin solidus that was 1⁄72 of a libra (Roman pound) of gold equal to a mass of 24 siliqua, where each siliqua (or carat) was 1⁄1728 of a libra. This is believed to be the origin of the value of the karat.
A piece of alloy metal containing a precious metal may also have the weight of its precious component referred to as its fine weight. For example, 1 troy ounce of 18 karat gold (which is 75% gold) may be said to have a fine weight of 0.75 troy ounces.
Troy mass of silver content
Fineness of silver in Britain was traditionally expressed as the mass of silver expressed in troy ounces and pennyweight, in one troy pound (12 troy ounces) of the resulting alloy. Britannia silver has a fineness of 11 troy ounces, 10 pennyweights, or about 95.83% silver, whereas sterling silver has a fineness of 11 troy ounces, 2 pennyweights, or about 92.5% silver.
- Gold as an investment
- Gold coin
- Silver as an investment
- Silver coin
- Platinum coin
- Colored gold
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