Collectible coins from the United States, such as the 25-cent (quarter) denominations, are often included in Proof Sets, which are specially minted to showcase high-quality finishes. The 2019 Proof Set includes various coins that are struck at the Philadelphia Mint, characterized by a mirror-like surface and sharp details. These sets typically contain a quarter representing each state, along with the other denominations in the set, making them desirable for numismatists and collectors alike.