Collectible coins encompass a variety of types, including patterns and trial coins, which are often produced as test pieces for new designs or minting technologies. In the United States, one significant year for such coins was 1870, when trial pieces were struck in Philadelphia to evaluate potential designs for future currency. The 2 and 25 denominations represent specific values that were tested during this period, highlighting the experimentation behind coinage and the evolution of American money. Collectors seek these rare items for their historical significance and unique characteristics.