Collectible coins can encompass a variety of types, including those issued by the United States of America, such as 25-cent denominations like the quarter. Proof sets, particularly the 2021 edition, contain specially minted coins with a shiny, mirror-like finish. These sets are produced for collectors and often feature enhanced designs and finer details compared to standard circulation coins. Collectible coins often hold significant numismatic value due to their rarity, condition, and demand among collectors.