The fineness of a precious metal object (coin, bar, jewelry, etc.) represents the weight of fine metal therein, in proportion to the total weight which includes alloying base metals and any impurities. Alloy metals are added to increase the hardness and durability of coins and jewelry, alter colors, decrease the cost per weight, or avoid the cost of high-purity refinement. 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% of other metals, usually copper, by mass. 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 or carats used only for gold. Karats measure the parts per 24, so that 18 karat = 18⁄24 = 75% gold and 24 karat gold is considered 100% gold.
Millesimal fineness 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 "14 k", "18 k", etc., which is currently used in the United States, and was standard in the United Kingdom prior to the 1970s. 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 for them are:
Platinum 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 850 750
Gold
999.999—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 in production; the Royal Canadian Mint regularly produces commemorative coins in this fineness, including the world's largest, at 100 kg. 999.9—four nines fine: Most popular. E.g. ordinary Canadian Gold Maple Leaf and American Buffalo coins. 999—24 karat, also occasionally known as three nines fine: e.g., Chinese Gold 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. 965: Thai standard for gold purity. Often considered equivalent to 23 karat in this context. 958—23 karat 916—22 karat: Crown gold. Historically common for bullion coins, and currently used for British Sovereigns, South African Krugerrands, and the modern (since 1986) American Gold Eagles. Standard for jewelry in some countries such as India. 900—one nine fine: American Eagle denominations for 1837–1933; used in Latin Monetary Union mintage (e.g. French and Swiss "Napoleon coin" 20 francs). 899—American Eagles briefly for 1834–1836. 834—20 karat 750—18 karat: Typical fineness for modern jewelry in most of Europe. Even in regions where higher purities are common, this is the maximum fineness used for gemstone-set jewelry, as the metal must be hard enough to robustly hold the stone in place. 625—15 karat 585—14 karat: Most common fineness for jewelry in the United States. While 14 karat is more precisely 583.3 fine, this level is not generally used in decimal fineness. 500—12 karat 417—10 karat: Historically, the minimum standard for gold in the US. As of August 2018, any marked karat is permissible. 375—9 karat: Minimum standard for gold in some of the Commonwealth realms: Australia, Canada, New Zealand, UK, etc. It is also the minimum in Austria, Ireland, Portugal and France. 333—8 karat: Minimum standard for gold in Germany after 1884. It is also the minimum for Denmark, Greece and Mexico. 042–1 karat: Legal minimum for gold in the US since the revision of the FTC Guides of August 2018.
Silver
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