The $20 double eagle coin is generally considered the rarest coin in the world. Illegal to own until only recently, the coin would be seized by Secret Service agents, and most copies have been destroyed.
Theodore Roosevelt was touring a museum when he was impressed by a series of ancient Greek coins. Roosevelt approached a sculptor to design various coins. The sculptor, Augustus Saint-Gaudens, died before completing the series, however. The one coin he finished was the $20 double eagle.
Why It's Rare
In 1933 Franklin Roosevelt decided that America should no longer take part in the gold standard and that all gold coins should be melted down. That included most of the $20 double eagle coins, but a few were not melted. They are now part of the National Numismatic Collection.
Until very recently, it was illegal to privately own a $20 double eagle coin. In 2002, however, the coin was put up for auction. It was estimated to sell for around $4 million, but it actually sold for more than $7.5 million.