Peaks & Valleys: Luster Explained

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Hidden under the flat surfaces of uncirculated coins lies a secret: jagged, microscopic ridges rivaling any Coloradan mountain landscape. Though mere microns across, these little striations play an outsize role in the coin’s appearance, giving us that shine we know and love: mint luster. 

The Basics 

Mint luster is the effect of light reflecting off a coin from mint-made imperfections on the surface, a phenomenon that can present itself in different ways. The U.S. Mint notes that luster can manifest as the “dull, frosty, or satiny shine found on uncirculated coins,” a glow that coins lose as they circulate. 

These surface “imperfections” are not forms of damage at all, but simply byproducts of the minting process. When a (non-lustrous) planchet is struck, it takes on the mirror image of the die under dozens of tons of force. The immense pressure causes the metal to behave almost like a liquid, flowing into the incuse portions of the die (i.e., the portrait, lettering, or other main devices). These flow lines on the planchet effectively etch themselves into the die each time a coin is struck, causing increasingly noticeable striations, which reflect light. This varies from coin to coin and mint to mint, as different designs, striking pressures, planchet characteristics, die polish, and other factors impact the flow lines and, in turn, the luster. 

To picture this, consider the mountain analogy. For most uncirculated coins, particularly those with classic “cartwheel” luster (which mimics the shape of a revolving bow tie), microscopic lines of metal emanate from the coin’s center, with the pattern nearly parallel. From one given angle of light, some “mountains” block the light (casting dark shadows), while others are in a prime spot to reflect it. As you tilt the coin, the light hits different mountains. Voilà—you have the cartwheel effect. 

Another way of picturing this is to consider a scratch or major bag mark, and how noticeable the color (i.e., light) difference is. Though scrapes are post-mint damage, it’s the same premise: a difference in elevation in the coin’s surface geometry catches light, thus appearing disproportionately bright at some angles. This effect is also visible by tilting designs with many parallel lines—for example, the eagle’s shield on Draped Bust dollars or many other lightly circulated U.S. coins. 

Mirrors & More 

Perhaps even more alluring than cartwheel luster is the coveted prooflike designation. Prooflike coins boast highly reflective, mirrorlike surfaces and deep contrast between the fields and devices. Reflectivity at 2 to 4 inches warrants a prooflike (PL) grade and deep mirror prooflike (DMPL or DPL) at 6 (for Morgan dollars). Try holding a book next to a coin to measure its reflectivity.

A well-polished die (which not all are) will, at first, strike highly prooflike coins. This effect diminishes over time as flow lines form on the previously flat die surface, resulting in a subtle satiny luster. As flow lines strengthen, a bolder, almost crystalline frosty luster forms, oftentimes synonymous with cartwheel luster. In essence, surface finishes typically progress from prooflike to semi-prooflike to satiny to frosty (though other types, like “orange peel” luster, exist too). 

Lost Luster 

Cleaning, perhaps the greatest sin in numismatics, is often mistakenly intended to restore luster or shine. In reality, abrasive cleaning can have the opposite effect since it effectively bulldozes the natural peaks and valleys. Once leveled or scratched, flow marks are gone for good. Cleaning cannot be undone, and surfaces cannot be restored, which explains why cleaning is so reprehensible to numismatists. (Note: Die polish lines can be confused with cleaning. Cleaning hairlines [oftentimes parallel] are embedded into the surface, rather than raised out of the surface like die polish or flow lines.) 

Dipping is a less destructive form of cleaning. Chemicals strip away a thin layer of metal and/or toning, which is a physical film on the surface. Though a single, short dip will hardly damage luster—and can even improve it by removing toning—repeatedly dipped coins can become dull. 

Grading 

Wear (circulated) and surface marks (uncirculated) steal the spotlight when it comes to grading coins. Still, luster plays a significant role, even for circulated pieces. In mint state, luster is one of the main components of grading, comprising approximately 15 percent of the overall grade at Professional Coin Grading Service. The company has stated in the past that coins with average luster cap out at Mint State (MS)-66, and coins with little to none cannot exceed MS-63. 

On the circulated side, the amount of luster remaining can differentiate between Extremely Fine (XF) and About Uncirculated (AU) grades, especially for weakly struck coins where wear is difficult to ascertain. The line between a nice AU grade and an MS one can be puzzling. The fields lose luster first, so look for areas of darkness where the microscopic peaks have been flattened by wear. As a coin continues to circulate, luster clings to the confines of lettering and the borders of devices longest. As general guidance, XF coins may have traces of mint luster while Very Fine coins almost never do. 

Conclusion 

Reading about luster, of course, can’t beat the experience of examining coins in hand and applying this knowledge as a coin detective of sorts. Next time you examine a coin, remember that luster is a lot more than simply shine. Coins truly do have a lot of depth, and this just scratches the surface.

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