Collectible coins, including those from the United States, encompass a wide range of denominations and historical significance. The 1-cent coin, or penny, is particularly popular among collectors. The 1991 Denver Mint issue features the Lincoln penny, made of copper-plated zinc. Collectors often seek specific years, mint marks, and condition grades to enhance the value of their collections. Rolls of these coins are typically gathered for both circulation and numismatic purposes, with coins from the Denver Mint being distinguished by a "D" mint mark.