Collectible coins, such as the 1981 Denver mint quarters, are part of a broader category of numismatics. These coins can gain value based on their rarity, condition, and historical significance. The 25c Roll, comprising quarters, often attracts collectors due to special mint marks, unique errors, or variations in design. In the case of the 1981 quarters from Denver, they can be found in various grades, influencing their market value and desirability among collectors.