The "grade" of a coin describes the wear and tear the coin has. Coins that are worn, scratched and dinged up have a lower "grade" than uncirculated, pristine examples. Coin grading ranges from poor (an unidentifiable coin, or a coin identifiable only because it is a 1 year type), to mint state (a coin that is nearly untouched). Generally, coins are graded by various "third party graders" such as NGC and PCGS which charge a fee and state their opinion of the coin.
The mintmark of a Morgan dollar is found on the back of the coin under the wreath. It is possible for a Morgan dollar not to have a mintmark, in which case the coin was minted at the Philadelphia mint. However, unless the coin grades exceptionally high by a third party grader, most 1921 Morgan dollars are sold for a little more than the bullion value of the silver because they are exceptionally common and silver is near historic highs. Today a 1921 Morgan dollar in average shape might be worth anywhere from $28-35.
If you know it's a 1921 "S" Morgan you found the mint mark
If it has a mintmark it's on the reverse just above the letters DO in DOLLAR.
The 1921 Morgan is the most common of all and regardless of mint mark, circulated coins sell for $14.00-$25.00 Mint state coins sell between $30.00-$40.00 and as all ways value depends on the grade.
The mint mark position on all Morgan silver dollars is located between the wreath and the DO in DOLLAR. No mint mark = Philadelphia S = San Francisco O = New Orleans (up to 1904) CC = Carson City (up to 1893) D = Denver (1921 only)
It's a 1921 MORGAN and in circulated condition is valued at $14.00-$19.00
1921 Morgan dollars unless they have an exceptional strike and are virtually perfect generally sell for about their silver melt value, about $25 at current melt value.
7-4-11>>> Of all the Morgan dollars the 1921 coins are the highest mintage most common. Values for circulated coins are $35.00-$41.00.
The mint mark (if it has one) on all Morgan (1878-1904 & 1921) silver dollars is on the reverse of the coin, just above the letters "DO" in dollar.
The mint mark position on all Morgan silver dollars is located between the wreath and the DO in DOLLAR. No mint mark = Philadelphia S = San Francisco O = New Orleans CC = Carson City D = Denver (1921 only)
1921 is the only year Morgan dollars were struck at the Denver Mint. No other dates could have a "D" mintmark. All 1921-D Morgan's have a tiny or "micro" mintmark.
I am not aware of a 1921 Morgan Dollar that carries a mint mark of "O." However, the 1921 series do carry mint marks of "D'" and "S." However, what is interseting about the 1921 Morgan Dollars is that they were produced by the US mint after a hiatus of 17 years - secondary to the The Pittman Act that required that all silver dollars be melted. More info - There could not be a 1921-O dollar because the New Orleans mint closed in 1909. 1921 was the only year that Morgan dollars were struck in Denver, because the Denver mint opened in 1906, two years into that hiatus.
I think you have your types reversed. Morgan dollars (named for their designer George T. Morgan) were minted from 1878 to 1904 and in 1921, while Peace dollars were made from 1921 to 1935. Please see the Related Questions for more information.