Collectible coins, particularly U.S. quarters, are often sought after by numismatists for their historical significance and potential value. The 1991 Denver quarter is part of the Washington Quarter series, which was first issued in 1932 in honor of George Washington. A roll typically contains 40 quarters, and the 25-cent denomination is popular among collectors due to its relatively low cost and widespread availability. Specific mint marks, like the 'D' for Denver, can impact a coin's value, with lower mintage and certain varieties being more desirable among collectors.