Since the United States Mint began producing gold coins in 1795, it has always used an alloy of both gold and copper. The primary purpose, originally, was to make the coins both larger and stronger. Pure 24 karat gold is extremely soft and vulnerable to wear, damage and deterioration. Even after being mixed with copper, gold is still softer than hard metals like nickel. Another reason for the copper blend was to make the coins bigger and easier to handle.

For much of the 19th and early 20th centuries, American gold coins contained a 90% gold 10% copper mixture. The United States Mint tried to blend the copper as evenly as possible, but sometimes concentrations of copper would remain present in the gold alloy. When that happened, U.S. gold coins would display obvious reddish-purple spots and streaks. The coloration can sometimes be extremely attractive, but in many cases these copper splotches can be highly distracting.

There is some debate as to whether copper spots should detract from a coin’s grade. Some argue that they are mint-made and do not represent damage or deterioration. Others, meanwhile, believe that copper spots can impair a coin’s eye appeal and thus should have a lowering effect on grade. PCGS and NGC apparently agree with the former opinion, as quite a few spotted gold coins have been awarded lofty grades of MS67 to MS69.

The tradition of adding copper to gold coins continues today in the American Gold Eagle program. To give the coins strength and added size, U.S. Gold Eagles have a 22-karat composition of 91.66% pure gold with the remainder consisting of copper and silver. As a result, a one-ounce Gold Eagle has a total weight in excess of an ounce, but still contains exactly one ounce of pure, fine gold.

While this unusual alloy does not bother most collectors and investors, some gold buyers prefer a coin with a higher purity. To compete with products like the 99.99% pure gold Maple, the United States Mint introduced the 24 karat 99.99% pure gold Buffalo in 2006. This was the first American gold coin to have ever been struck without some amount of copper in the alloy. However, the coins were extremely susceptible to damage and could not be delivered to buyers in tubes like Gold Eagles. Thus, the Mint was forced to package the coins in protective plastic sheets to avoid contact marks.