Many numismatists, dealers and collectors consider unique coins to be the most coveted, prized and valuable. Many of these unique coins were documented by the U.S. Mint or one of the Mint Branches, while others have no documentation but there are logical assumptions as to how and why they were struck.
Some of the most famous U.S. coins were struck under mysterious circumstances and the story behind their production adds to their rarity, prestige and value. Examples are coins struck in proof where only one example is known, a special specimen striking struck with polished dies, while others are patterns, trial strikes, or mint error coins.
Here is the only known 1891-O Seated Liberty Dime struck in PROOF! Authenticated and certified by NGC as a Proof 66, this unique Branch Mint proof is a world-class rarity.