Most coin collectors are familiar with the term "mint luster" (or lustre), but many are at a loss to describe it and it is likely that most do not know the cause of it. The American Numismatic Association’s "Official A.N.A. Grading Standards for United States Coins" and James F. Ruddy’s "New Photograde", the two most commonly used grading guides for U.S. coins, only devote two sentences each to define the term. The presence or absence of mint luster is an important characteristic of a coin’s surface, and it is to the collector’s advantage to be able to recognize it and to understand its origin. 

Mint luster is formed during the minting process. The surface of the die that strikes a planchet has slight irregularities. When the die strikes a blank planchet, metal flows into the recesses of the die and outward toward the rim. The irregularities in the die are reproduced on the surface of the coin. Moreover, the flowing metal scores the surface of the die with faint lines called flow lines. These flow lines become more pronounced as the die is used. The flow lines cause the formation of radials on the surface of the coin which fan out from the center and are most visible near the edge but are also visible near the raised design, letters and date. The irregularities transferred from the die as well as the radials are readily observed under a microscope. The radials appear as tiny ridges and valleys. When light strikes the surface of the coin, it is reflected in all directions by the irregularities and radials, giving a diffuse appearance to the coin’s surface. This appearance is sometimes referred to as mint frost. 

A Coin having mint luster will show the cartwheel effect. The cartwheel effect is similar to the effect shown by a rapidly rotating stagecoach wheel. The wheel will appear to be rotating slowly in a direction that is opposite to the actual direction of rotation. When a coin having full mint luster is tilted back and forth in the presence of a light source, the direction of the diffused light reflected off of the surface will change from clockwise to counterclockwise as the direction of the tilt is changed. The writer has found that the cartwheel effect is easier to observe if the light source is off to the side rather than directly over the surface of the coin and only one light source or direction is present. Having more than one light source coming from different directions will produce more than one direction of reflection of light, causing the cartwheel effect to become obscured. It can sometimes be difficult to observe mint luster on a coin at a coin show. The brightness of the dealer’s lamp will make the cartwheel effect difficult to observe, especially if it is directly over the coin. Sometimes it is helpful to hold the coin below the top of the dealer’s table to block out the dealer’s lamp and rely on the overhead lights. (This should be done in a proper manner so that the dealer is not aroused to suspect the possible theft of the coin.) 

Not all coins will exhibit the same degree of luster. The amount of luster present on a coin will vary depending on the extent of the irregularities and flow lines that were present on the dies used to strike it. As discussed above this extent changes over the lifetime of a die. Coins struck from new dies do not show as much luster as coins struck from older dies. Proof coins do not show mint luster. These coins have highly reflective mirror like surfaces. This is because the dies used to strike them are highly polished and do not have the usual irregularities and flow lines. Coins struck for circulation from new dies (sometimes called "first strikes" or some similar term) often have prooflike surfaces. This phenomenon is commonly observed on uncirculated low mintage coins. An example is quarters minted in the 1880’s, where the total mintage was under 16,000 pieces per year. Sometimes it is difficult to distinguish whether a particular coin from this era is a proof or a business struck piece. Morgan dollars and other type coins having prooflike surfaces often command a premium in price in the marketplace. Cameo prooflike coins (frosted devices and prooflike fields) often command even higher premiums. A weakly struck coin does not show regular luster on the raised flat areas because these parts of the coin’s surface did not touch the dies during striking. A different texture dull luster will be present on these areas which can be seen under a microscope. Mint luster is easier to observe on large coins than on small coins. This is because these coins have more and larger radials (flow lines). Therefore, when learning to recognize the presence of mint luster on coins, these are good coins to practice on. Toning will tend to hide mint luster. Luster on a deeply toned coin may not be visible to the naked eye, but it can be observed under a stereo microscope. The composition of a coin as it relates to mint luster is not an important factor. However, it appears to the writer that nickel coins do not show mint luster as well as silver, copper and gold coins. 

An uncirculated coin will exhibit full mint luster. As a new coin is circulated the peaks of the radials on the high points of the design will bend and the gradually wear. This impairment of the luster will cause the cartwheel effect to no longer be perfect. As the coin is tilted, there will be a break in the cartwheel effect in those regions of the coin that have been worn, i.e. the highpoints of the coin. These regions will show a slightly different color than the rest of the coin and will appear duller than the lustrous regions. Coins graded almost uncirculated (AU) are sometimes dipped to remove the dullness. This causes the dull areas to become shiny. As the amount of wear increases, the extent of the break in the cartwheel effect also increases. Coins that grade extremely fine (EF or XF) will show the cartwheel effect near the edges and in the protected areas. A slight break in the cartwheel effect can be difficult for an inexperienced collector to observe. Many coins grading AU may appear to give full cartwheel effect, when in actuality the cartwheel effect is present mainly in the fields and protected areas of the coin. It is important to observe whether or not the cartwheel effect is present on the devices (design elements) as well as in the fields. This can be difficult to ascertain near the center of the coin, but this can be done with experience. Examination under a stereo microscope is very valuable for making this determination. A pocket light scope having a magnification of 30X is very useful for doing this at coins and at coin shops. At a bare minimum a good magnifying glass should be used. It is recommended that any "uncirculated" coin be examined for full luster before it is purchased. This is particularly important if the coin is expensive.

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