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.
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.
Reproduced with permission from The Official American Numismatic Association Grading Standards for United States Coins, 6th edition, © 2005 Whitman Publishing, LLC. All Rights Reserved.