Collectible coins refer to rare or special coins that are sought after by numismatists and collectors. Among these, the United States Twenty Dollar gold coins, specifically the 1908 patterns and trial coins minted in Philadelphia, are notable. These coins were created as experimental pieces before the final design was approved for circulation. Patterns and trial coins often have limited mintage, making them highly valuable and collectible. The 1908 examples are particularly sought after due to their historical significance and unique design features.