An 1881 Morgan Dollar with Original Mint Luster. Image Courtesy of: Heritage Auction Galleries,
Mint Luster on a coin's surface is the sheen or reflective qualities that are produced during the minting process. When a coin is struck, metal flows from the center of the coin towards the outer edge. This process produces microscopic striations (very tiny parallel grooves) that reflect light back to the viewer in a unique crossing pattern. As the coin is tilted under a light, this crossing pattern will spin around the coin.
This is known as the cartwheel effect.
Mint Luster is extremely delicate and can be damaged or destroyed by friction from other coins or objects that rub across the surface of the coin. It cannot be repaired or replaced after it is destroyed. Any attempt at restoring mint luster is considered "Coin Doctoring" or altering the surface of a coin with the intention to deceive a buyer.
Official A.N.A. Definition
Luster, or mint bloom or frost, is one of the most important aspects of grading in the Mint State category. All other things being equal, a coin with rich, deep mint luster is a better candidate for a higher grade than is one with a dull or lifeless luster. One rule does not fit all; luster can vary from type to type, and examination of a wide selection of coins in the marketplace is the best way to gain expertise in this vital feature.
Affect from Coin Cleaning
Many people think that a clean and shiny coin is worth more than a dull and toned coin. Unfortunately, most coin collectors and numismatists prefer original surfaces over those that have been cleaned. Harsh and abrasive cleaning techniques that use acids and abrasives remove the mint luster that has been imparted on the surface of the coin during the minting process.
Coins that have been improperly cleaned will exhibit a muted or subdued luster. Uncirculated coins that still have their original surfaces will exhibit a flashy and bright mint luster that will dance across the coin as it is rotated and tilted.
Grading Uncirculated Coins
Original mint luster is one of the key components when determining the grade of an uncirculated coin. A coin that has original and flashy mint luster will have a better eye appeal than one that has had its luster muted or damaged.
Coins that grade About Uncirculated or below will usually have areas on the coin where the mint luster is broken or missing. This is one of the key factors that a coin is not a mint State coin. If a coin has details that indicate it has been in circulation, and yet it exhibits some sort of flashy mint luster, this is evidence that the coin has been doctored in an attempt to make it look uncirculated. This is a form of alteration and it is considered deceptive by coin collectors.
Examples of Mint Luster
If you are still having problems recognizing what original mint luster is, visit your local coin dealer or a coin show. While you are there ask a dealer to see some uncirculated Morgan silver dollars.
It will be easier to recognize on coins that are not toned. Ask him or her to show you what original mint luster looks like on one of these big silver coins. You will be able to see the distinct difference immediately.
Reproduced with permission from The Official American Numismatic Association Grading Standards for United States Coins, 6th edition, © 2005 Whitman Publishing, LLC. All Rights Reserved