In 1921, the U.S. Mint produced the last of the Morgan silver dollars and the first of the Peace silver dollar coins. 1921 was one of the highest production years for Morgan dollars, so there are many of these coins remaining, making them more affordable for collectors. The Mint produced fewer of the Peace dollars, so these coins have a higher value.
The Mint produced Morgan dollars from 1878 until 1921. In 1921, the Philadelphia, Denver and San Francisco mints produced these coins. Collectors use a mintmark above the "d" and "o" in the "dollar" label to determine the mint that made it. The Philadelphia mint did not place a mark on coins it produced, while Denver placed a D-mark, and San Francisco marked its coins with an S-mark.
The government produced about 1 million Peace dollars in 1921, all at the Philadelphia mint. These dollars do not have a mintmark, but display a high-relief design that the mint abandoned later. These high-relief coins offer a very distinct pattern, but producing them was impractical, and the mint moved to a low-relief design for later years. Because this was the first production run for these coins, a large number remained uncirculated, increasing the collector's value of the coin.