This photo provided by Stack's Bowers Galleries shows a rare $20 double eagle gold coin from 1870, which sold for $1,440,000 at an auction. (AP) A rare $20 "double eagle" gold coin minted in 1870 ...

Other historic US coins have fetched vast sums at auction too. A rare 1794 silver dollar believed to be one of the first – if not the first – made by the US mint sold for $10 million in 2013.