No, they're not rare. A coin without a mint mark means it came from the Philadelphia mint. Coins dated 1955-67 do not have mint marks even though they were minted at all 3 mints.
Philadelphia first used the P mint mark on nickels made from 1942 to 1945. The P mint mark was dropped after WW2 and not used again until 1979 when it was put on dollars. In 1980 Philadelphia began to use a "P" on all other denominations except the cent. Cents from Philly continue to be made without mint marks because West Point and San Francisco occasionally strike small numbers of cents when demand warrants. These coins do not have mint marks either, so they're indistinguishable from Philadelphia cents and thus do not become instant rarities.
If your coin does not have a mint mark then it means it was minted at the Philadelphia mint. These coins are usually not as rare. However recently P mint marks were added to coins to indicate that they were made at the Philadelphia mint.
The letter is known as the mint-mark and tells where the coin was made. A coin having a P mint-mark (or no mint-mark on some coins) was made in Philadelphia, a coin having an S Mint-mark was minted in San Fransisco, a coin with a D mint-mark was minted in Denver.
The mint mark is located on the reverse [tails] side of the coin at the bottom below the wreath. If there is no mint mark then the coin was minted at the Philadelphia Mint.
There is not always a mint mark on coins. If your coin does not have a mintmark it means that it was made at the Philadelphia mint in Pennsylvania.
No US coin bears an "F" mint mark
Mintmarks identify which mint the coin was made at.
It depends on the type of coin the date and denomination on were the mint mark is located
The mint mark of the Denver Mint (shown as a "D") and the San Francisco Mint (shown as "S) on the Eisenhower Dollar is located on the obverse (heads) side of the coin directly beneath the bust of Eisenhower. If there is no mint mark there, then the coin was struck at the Philadelphia Mint and in this mint did not place a mint mark on the coins struck there until 1979.
If it is a proof then it will have the mintmark "S". If your coin doesn't have the mintmark "S" then it is just a normal coin.
The earliest the coin was produced by the US mint, the more rare it probably is. However, the mint state would also contribute to the coin's value.
A mint mark is a smaller letter or letters representing the mint facility the coin was struck and are found in various places on the coin.
"cc" is a mint mark. The Mint mark shows where the coin was made. "CC" stands for Carson City, Nevada.