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 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 = = 75% 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 "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. Here are the most common millesimal finenesses used for precious metals and the most common terms associated with them. 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 925 900—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 currently produced; the Royal Canadian Mint regularly produces commemorative coins in this fineness, including the world's largest at 100 kg.