How a Coin is Graded
Numismatists grade coins according to a 70-point Mint State (MS) scale, with roughly MS-65 and up qualifying as a gem uncirculated coin. They are trained to make an evaluation based on characteristics such as strike, luster, wear and number of flaws. According to Mitch Hight of the American Numismatic Association, "grading is a subjective art," meaning there may be minute differences in opinion between one appraiser and the next.
Uncirculated Coins
Uncirculated coins are the highest-graded -- MS-60 and above -- and therefore highest-valued, aside from limited "proof" coins, which have their own scale. Uncirculated coins have never been used as currency and, as a result, have not become dull or worn through contact with human hands. Given the relative impossibility of verifying this, however, the American Numismatic Association acknowledges that "a more practical definition is a coin that shows absolutely no trace of wear."
Gem Brilliant Uncirculated Coin
Even among uncirculated coins, there are degrees of perfection. Numismatists vary on the adjectives they apply to the highest-graded coins, but terms like "brilliant" and "gem" are always reserved for the finest specimens graded over MS-65. Brilliance refers to the coin's luster or shine, while a gem coin features almost no discernible flaws. Austin Rare Coins &Bullion indicate that most coins of this quality are found in museums or in the hands of very serious collectors due to their scarcity and value.
Buying or Selling
If you are interested in buying a highly-graded coin, make sure you are dealing with a reputable seller, as the factors which separate a low-graded uncirculated coin and a gem are often imperceptible to the untrained eye. Make sure the grading has been performed by a trustworthy third-party, such as the Professional Coin Grading Service or Photograde Coin Institute. If you are looking to sell a coin you believe to be in pristine condition, it is advisable to contract one of these organizations to have your coin's grade verified.