What This Coin Looks Like (Obverse, Reverse, Mint Mark Location, Special Features, etc.): One of the rarest and most valuable US coins is the Type 1 1853 O Seated Liberty Half Dollar - without arrowheads at the date and no rays on the reverse. Only 4 of these coins of this variety are known to exist. 1853 was a transitional year for the seated liberty half dollar when the two arrowheads were introduced on both sites of the date on the obverse and rays were added around the eagle on the reverse. It's possible that this coin is so rare because the New Orleans Mint may have begun producing a few 1853 examples before the new coin dies (with the arrows and rays included with the new design) arrived from the Philadelphia Mint Headquarters.
The most recent Howell piece example of this coin was discovered by accident in 2012 in Very Good (VG-8) condition and was sold for $218500 at auction. It's possible that more still exist somewhere! Source of example image is courtesy of Stack's Bowers Galleries'.
Coin Value Chart: Typical Coin Prices, Values and Worth in USD based on Grade/Condition