As with other precious metals, gold is measured by troy weight and by grams. And when it is alloyed with supplementary metals the term carat or karat is used to specify the amount of gold present, with 24 carats being pure gold and lower ratings being proportionally less.
The purity of a gold bar can also be written as a decimal figure ranging from 0 to 1, known as fineness, such as 0.995. For example, 1 troy ounce of 18 carat gold (which is 75% gold) may be said to have a fine weight of 0.75 troy ounces.
Carat and gold price
Carat is a measure of the purity of gold and platinum alloys. One carat is one twenty-fourth purity by its weight. Thus 24-carat gold, also occasionally known as three nines fine, is pure gold (99.99%); while 12-carat gold is 50% purity. In the United States and Canada, the word karat is typically used for the measure of purity, while carat is referring to the measure of mass.
The carat system is gradually more being complemented or superseded by the millesimal fineness system where the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.
The most frequent carats used for gold in bullion, jewellery making and goldsmith are:
24 carat (millesimal fineness 999), 22 carat (millesimal fineness 916), 20 carat (millesimal fineness 833), 18 carat (millesimal fineness 750), 16 carat (millesimal fineness 625), 14 carat (millesimal fineness 585), 10 carat (millesimal fineness 417) and 9 carat (millesimal fineness 375).
The historical gold price
Historically gold was used to back currency in an economic system recognized as the gold standard where a certain weight of gold was given the name of a unit of currency. For a long period, the United States government set the value of the US dollar so that one troy ounce was equivalent to $20.67 ($664.56/kg), but in 1934 the dollar was revalued to $35.00 per troy ounce ($1125.27/kg). By 1961 it was becoming harder to uphold this price, so a pool of US and European banks agreed on manipulating the market to stop further currency devaluation against increased gold demand.
In March 1968, economic conditions caused the collapse of the gold pool, and a two-tiered pricing scheme was established. Gold was still used to settle international accounts at the old $35.00 per troy ounce ($1.13/g) but the price of gold on the private market was allowed to rise and fall.
This two-tiered pricing system was discarded in 1975 when the price of gold was left to find its free-market level. Central banks still hold historical gold reserves as a store of value even though the level has generally been declining. The biggest gold depository in the world is that of the U.S. Federal Reserve Bank in New York.
Ever since 1968 the price of gold on the open market has ranged widely, with a record high $1889/oz ($59,600/kg) in September 2011, to a low $252.90/oz ($8,131/kg) in June 1999 (London Fixing).