It is often thought of as one of the most significant and well-known patterns in the history of the U.S. Mint. The basic design slightly modified the similar $20 "Liberty Head" Double Eagle, which was designed by James B. Longacre and minted from 1849-1907.
In 1877, famed Chief Engraver of the Mint at the time, William Barber, designed the coin. William Barber also designed several other pattern coins such as the "Amazonian Quarter", Twenty Cent Piece, and the famous Trade Dollar. The coin was designed to weigh roughly 2.5 ounces and be made of solid gold. Had it been made for circulation with the general public, the coin would have been the highest valued gold coin ever made at the time, with a face value of fifty U.S. Dollars. As it is a pattern, it was never struck for circulation and all other known presentation versions were made of copper or sometimes various gilded metals. Only two examples were actually struck in gold, and today both reside in the Smithsonian.
However, the coin did resurface in the form of a $50 commemorative coin released in 1915 to commemorate San Francisco. Some of the coins were octagonal, others round, making it the first and only time the United States Mint has ever released a coin that was not round.
Famed designer George T. Morgan thought of an early possible design for a $100 full Union coin, should the Half-Union ever be a success. When the mint found the design of a 2.5 ounce solid gold coin completely infeasible, the idea of a complete Union was discarded altogether and was forgotten about, often only thought of as a possibility. However, around 2005, Morgan's original sketches were discovered and published so the Numismatic world could see what could have been. Private mints have since struck fantasy pieces of Morgan's design for collectors, in both silver and gold. 
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