By Greg Reynolds for CoinLink …..
Only one 1870 San Francisco Mint half dime is known to exist. It was on display on October 27 & 28 at the first CoinFest, at the East Greenwich (CT) Civic Center. Please see my separate article about the CoinFest event. Most collectors have either never heard of the 1870-S half dime or do not know much about it.
The most famous coins are not the rarest.
Almost all collectors know about 1909-S VDB and 1914-D Lincoln cents; 1877 Indian cents; 1913 Liberty nickels; 1894-S, 1895-O and 1916-D dimes; 1901-S and 1913-S Barber and 1916 and 1918/7-S Standing Liberty quarters; and other key dates in series that are avidly collected. Moreover, 1804 silver dollars are the most famous of all coins. As 15 1804 dollars are known, there are quite a few other coins that are rarer.
Most collectors are not familiar with the rarest U.S. coins.
There are three, privately owned U.S. coins that are each unique. Before itemizing the three, it makes sense to emphasize that I am referring to privately owned coins. There are unique pieces in the Smithsonian, and these are more complicated, for at least two reasons. Most unique pieces in the Smithsonian are patterns rather than true coins. Those unique pieces that are definitely true U.S. coins are not necessarily distinct dates. The issue of whether a particular die variety is a separate date is often controversial. There is no doubt that the three privately owned unique U.S. coins are true coins and are distinct dates.