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Crown gold is a 22 karat (kt) gold alloy, crown coin, introduced in England in 1526 (by Henry VIII) to replace the earlier gold sovereign coins which were made from the softer 23 kt gold and which thereby had invited both deliberate filing and also non-intentional wear. The 22 kt standard has been used for British gold coins ever since this change which is 91.667% gold (or 22 parts out of 24).
Alloying metals in England and the United Kingdom 
The alloying metal in England was and is traditionally copper-only, although a component of 1.25% silver replacing the same weight of copper, as a softening agent, was briefly used in gold sovereigns in the single year of 1887, for better production of an image of Queen Victoria for the Golden Jubilee of her reign. Crown gold with copper as the only alloying metal other than gold, remains the standard used in the British gold sovereign, which is still minted.
Alloying metals in the United States 
In the United States, gold circulating coins were minted in 22 kt. gold alloy of crown gold purity, until 1834. These pre-1834 U.S. coins differed from their English counterparts in containing a significant component (about 6%) of silver.
In 1834, the fineness of U.S. coin gold was decreased from 22 kt to 0.8992 fine (21.58 kt) to discourage coin melting. In 1837, all silver was removed from U.S. gold coins and the gold content increased slightly to 0.900 fine (21.60 kt exactly). This 90% gold-copper alloy continued in the U.S. from 1837 until gold coins were removed from circulation in the U.S. in 1933.
Pure gold bullion coins departing from crown gold or alloy gold standards 
Before the mid twentieth century, all gold coins were intended to circulate as money and therefore contained either 90% gold or a 22 kt crown gold type alloy. Bullion coins were subsequently designed as collectable and not for circulation, which could be much more pure, since wear-problems were not an issue. Some of these gold bullion coins are pure gold of 24 kt, 0.999, 0.9999, or even 0.99999 fine (see Canadian Maple Leaf).
Return to crown gold for some bullion coins 
The South African Krugerrand, first produced in 1967, and the first of these modern coins, was produced in the exact traditional crown gold recipe of 22 kt gold (remainder copper), because it was originally intended to circulate as coin currency. The Krugerrand is now considered primarily a bullion coin, and as such has no face value imprinted on it. A few other gold bullion coins never intended for circulation have also been produced for purely historical reasons in the traditional crown gold purity, including the gold sovereign (in exact gold/copper historical ratio), as well as now the American Gold Eagle series of bullion coins, the latter which again now follows the original (pre-1834) 22 kt or 0.91667 fine gold standard (although its silver content of 3% is somewhat different from that of any coin the U.S. had ever previously produced).