If you covered the date of a $20 Saint Gaudens Double Eagle and showed it to an expert numismatist, he or she could probably guess the date with excellent accuracy. How is that possible? The $20 Saint design remained largely unchanged from 1908 through 1933, but the texture, color and luster of the coins vary quite a bit year to year (and from mint to mint). A 1908 Philadelphia issue, for instance, is likely to have satiny luster with a slightly hazy appearance. A 1915 San Francisco coin is apt to be grainy in texture. Meanwhile, 1927 Philadelphia “Saints” usually exhibit booming cartwheel luster. Why do these differences exist?
One factor is how long the dies were used. If the dies were constantly replaced and never given a chance to deteriorate, the coins are more likely to have a flashy, proof-like appearance. As dies wear down and their surfaces become more granular, the coins struck from these dies become more satiny. Advanced die wear and damage can also result in visible die cracks—these are commonplace on certain coinage issues.
Another contributor is the source of the metal. United States gold coins look notoriously different from mint to mint. The Charlotte and Dahlonega Mints, for example, used locally sourced gold almost exclusively. As a result these coins tend to have unusual (but completely natural) pink and green tinges that are much less likely to be seen on Philadelphia issues. This is due to the presence of impurities in the locally extracted metal.
Sometimes, coins have unusual appearance because the dies were sloppily prepared. The 1921 Morgan Dollar, for example, exhibits weak luster and shallow detail impression due to how the dies were made. Sometime between 1904 and 1921, it was (incorrectly) assumed that the Morgan Dollar was going to be discontinued. Therefore the US Mint discarded the master Morgan Dollar dies. When more coins were unexpectedly needed in 1921, the Mint was forced to create new working dies from old transfer hubs dating back to 1878. The result is that 1921 Morgans have a distinctly bland appearance compared to their 1878-1904 counterparts.
Strike is another variable from mint to mint. Going back to the Morgan Dollar series, most San Francisco issues are perfectly struck while New Orleans issues are often laughably flat. If the highest points of Liberty’s hair are weakly struck, there’s an excellent chance there is an O mintmark on the reverse. Charlotte and Dahlonega gold coins were also made in a slipshod manner; very few of them display complete detail.