FYI there was a valuable post [gone now] in the gold section yesterday by Bron Suchecki about this. Have a look at this Perth Mint gold nugget, purity 9999. http://forums.silverstackers.com/message-380375.html#p380375 The discolouration in this case, is not actually a copper stain, but rather a silver stain. Bron said it was because in the old days at the Perth Mint they used the same equipment to fabricate both gold & silver coins, and sometimes tiny particles of the silver would be pressed into the surface of the gold coin. These days however, the gold and silver coins are fabricated on completelty separate machinery, hence this type of discolouration shouldn't be found on the more recent coins. Copper however, is probably the cause of the stains that you can find in many cases of <22K gold; eg. Kruggerands, which is 91.67% gold and 8.33% copper.
The thread has been restored http://forums.silverstackers.com/message-387225.html#p387225 after some cleansing by goldpelican.
