“AU” (About Uncirculated or Almost Uncirculated) is perhaps the best value among the major numismatic grades. AU coins are extremely close to being Uncirculated—and often have the visual appearance of a Mint State coin—but exhibit extremely minor signs of wear. Most AU coins retain a substantial amount of luster and virtually complete design details. Many can be described as “arm’s length gems,” i.e. having the initial appearance of an MS 65 coin but the technical qualities of an AU 55-58.
The primary difference between the grade Extremely Fine (XF) and About Uncirculated is typically the amount of luster. There are many XF coins that have “AU Details” but are downgraded to XF for being dull, lackluster or subdued with heavy patina. AU coins are expected to have virtually complete design detail with only the slightest signs of circulation. XF coins, by comparison, will show more obvious deterioration with the highest points appearing flat from wear.
Prior to the advent of PCGS and NGC, the grade About Uncirculated was a broad term. Little distinction was made between coins on the higher and lower ends of the spectrum. Nowadays, grades of AU 50, 53, 55, and 58 exist to differentiate more specific levels of circulation within the category. The line between AU 50 and 53 can be somewhat fuzzy; some numismatists have argued that these two grades are redundant. Coins graded AU 55 are sometimes called “Choice About Uncirculated.”
On the highest end of the AU range, the term “slider” is sometimes used for coins which show only the slightest traces of wear. Perhaps they were indeed slid across a table once or twice, incurring only a bit of friction, but even a slight rub can be enough for a coin to no longer qualify as MS (mint state). It might be said that an AU 58 coin is a MS 63 coin with a few traces of wear.
AU coins often prove to be great values for collectors: they have the general look and appearance of an uncirculated coin, but lack the premium price tag. It’s not unusual for a coin to double, triple or even quadruple in value from AU 58 to MS 61. For this reason, many collectors specifically assemble complete collections of AU 58 coins to save on cost while not sacrificing appearance. Uncirculated coins might be more valuable than their AU 58 counterparts, but often AU 58 coins are more liquid and avidly sought.