A fascinating, but 'not well known' sector of numismatics are Pattern Pieces.
What is a Pattern? Pattern pieces consist of numerous designs and experimental pieces which were made by the U.S. Mint in an effort to improve the nation's coinage. Basically, a pattern piece is a coin that was struck as a recommendation example for a new coin series, intended to show the Mint and Treasury Department officials or members of congress to evaluate. They were struck as an artist's idea of what a particular coin series design should look like.
If the pattern was accepted, it became a regular familiar issue. Whether accepted or rejected, pattern pieces were NOT intended to be sold, but before 1887, many were released from the mint, sold and traded to collectors, or given to government officials, and became available for the collecting society. During mid-1887, the Mint implemented strict regulations prohibiting the sale and distribution of U.S. Pattern pieces.
There are over 2000 distinct varieties and most of these are considered very rare and therefore expensive. It would be virtually impossible for any collector to assemble a complete set of U.S. Pattern Pieces. If you ever have the chance to purchase even 1 pattern piece, I recommend adding it to your collection. Studying patterns can not only give collectors a wider range of their numismatic knowledge, but they also learn about U.S. history, politics, 19th century artists and society, an extension of the Minting Process, and a true realization of what 'rare' really is!