999.5: what most dealers would buy as if 100% pure the most common purity for platinum bullion coins and bars.
Here are the most common millesimal finenesses used for precious metals and the most common terms associated with them. 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. 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. Many European countries use decimal hallmark stamps (i.e., "585", "750", etc.) rather than "14 k", "18 k", etc., which is used in the United Kingdom and United States. For example, an alloy containing 75% gold is denoted as "750". 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. Karats measure the parts per 24, so that 18 karat = 18⁄ 24 = 75% and 24 karat gold is considered 100% gold. 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. Sterling silver contains 92.5% silver and 7.5% of other metals, usually copper, by mass. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. 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.
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.
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