A single gold impression of the 1849 Liberty Double Eagle is known. Technically a pattern or experimental coin, it represents the first double eagle struck by the United States Mint.
The coin has never traded publicly, but was sent directly to the Mint cabinet in Philadelphia after its mintage, presumably on December 22, 1849. In 1909, J.P. Morgan was reported to have offered a sum of $35,000 for the coin, an extraordinary amount for the time.
Currently, the single known 1849 Double Eagle resides within the National Numismatic Collection at the Smithsonian Institution. One or two more gold impressions are supposed to have been struck, but none of these are currently located. If one were to appear again after 150 years it would rank amongst the greatest numismatic discoveries of all time, and it could very well be the most expensive coin in the world.
A single brass impression was also known in the 19th century, struck at a later date, but that specimen has not been seen since 1892.