A Guide to Toned Gold Coins January 04 2015
A quick chemistry note here: what coin collectors call "toning", a discoloration of a coin's surface, actually encompasses a wide range of chemical reactions between the surface of the coin and outside elements. For example, the "rainbow" toning seen on many Morgan dollars is the result of a reaction between silver and environmental sulfur, causing a silver-sulfide film to form on the surface of the coin. The refraction of light through the silver-sulfide film is what creates the vibrant coloration. Gold can also react with sulfur, though gold-sulfide film does not typically generate the same multicolored refraction of light.
Additionally, most gold coins are not pure gold—they are commonly alloyed with a small amount of copper. Circulating U.S. gold coins have generally been produced in a 90% gold - 10% copper alloy. Copper is more reactive than gold and even silver, and a small percentage of copper in a gold coin can lead to toning. Toning becomes more and more uncommon as the alloy increases in purity, and is only rarely seen on modern .9999 fine gold bullion coins (effectively pure gold). But, contrary to some skeptics' beliefs, even .9999 fine gold can tone. The color change may be very subtle, but it does exist. Just take a look at these .9999 fine modern gold coins produced by the U.S. Mint—there is a minute but definite darkening on certain areas of the coins (Martha Washington's shawl, the side and head of the buffalo).
So what types of toning can we expect to see on gold coins? If you're hoping for vibrant rainbow or textile toning, you may be disappointed. Gold toning often expresses itself in very subtle patina changes. The most common form of natural gold toning is a deepening of color from yellow to orange. If the toning progresses further, reds begin to emerge, and in the most vibrantly toned coins, you can even see some deep purples. Light rainbow toning is extremely rare, though it has appeared on a few Saint-Gaudens double eagles (and cleanly graded as natural toning by NGC or PCGS). But for the vast majority of toned gold, you can expect to see colors within the yellow-red-purple spectrum.
Rarity and Value of Toned Gold
For the collector interested in acquiring a few gold coins with toning, there's good news! The premiums attached to attractively-toned gold are much smaller than the premiums placed on, say, rainbow toned Morgan dollars. One contributing factor is the high starting value of gold coins—while many numismatists are willing to pay even a 3X premium for a high-end toned silver dollar (let's say $180 vs. $60), very few would be willing to apply the same multiplier to a Saint-Gaudens Double Eagle with a book value of $1500. It's the same reason why doubled dies and other die varieties drive much less of a premium in gold, compared to copper and silver coins. But the other reason that toned gold doesn't demand a huge premium is that there simply isn't a large market for it. Many gold collectors are more attracted to bright yellow, lustrous mint state (AKA "investment grade" gold) coins than toned gold, and many others probably remain unaware of the fact that toned gold even exists!
Gold coins with original toning represent a much smaller portion of the overall gold coin market than do silver coins or even copper-nickel coins. Part of that is because of the previously mentioned fact that gold is less prone to toning than other metals used in coinage. But another suspected factor is that gold coins have been more prone to dipping over the years. "Dipping" is the process of immersing a coin in a mildly acidic solution, effectively stripping off any toning/patina and leaving a bright, untoned coin. Dipping was used heavily on both silver and gold coins by many collectors throughout the 20th century. Dipping continues to be practiced today, though there is now a greater market appreciation for coins with original patina. When performed carefully, dipping can make a coin appear to be in a higher grade. Gold coins were even more susceptible to being dipped during the 20th century, due to their higher value and the correspondingly larger benefit to securing a high grade.
Below are two examples of an early U.S. gold coin, the "Turban Head" gold eagle. The coin on the left (from the fantastic Coin Rarities Online gallery) has not been dipped, and has an original patina. Contrast its deep orange color with the unnaturally yellow color of the coin on the right (same series, different denomination), which has been dipped at least once in its 200-year life.
Toned U.S. Gold Coins
Toning can be found on all series of U.S. gold, from the very first Turban Head gold coins onward. Unfortunately, due to the high rate of dipping, it is often difficult to find old gold coins with original deep-golden toning. Many 18th and 19th century coins are unnaturally bright for their age, visible evidence of dipping and potentially cleaning. The more dramatic red and purple advanced-stage toning is even scarcer, but does seem to be slightly more prevalent on Indian Head quarter and half-eagles. It is possible that the higher number of toned Indian Head gold coins (not including the $10 eagle) is related to the particular treatment that planchets for those two series of coins received. As the first U.S. coins with an incuse design, the quarter and half-eagles proved to be difficult to strike at each of the Mint facilities where they were produced (Philadelphia, Denver, and San Francisco). Eventually, one of the mint superintendents devised a solution of having the planchets shaved slightly before striking. This relieved some stress on the coining press, allowing the design to be struck with full details. My hypothesis is that the unique treatment of Indian Head planchets somehow made the coins more prone to toning - it is strictly conjecture, though.
Toned Ancient Gold Coins
As you might imagine, a coin left buried for thousands of years will undergo some dramatic changes in its appearance. And if that coin sits buried in a sulfuric environment, the toning that results can be extraordinary. The most famous example of this is the Boscoreole Hoard, a cache of over 1000 Roman gold coins buried under volcanic ash from the eruption of Mount Vesuvius. The gold aureui lay buried and undisturbed from the time of the volcano's eruption in 79 A.D. all the way up to 1894, when they were finally unearthed. Volcanic ash is extremely sulfuric, and the sulfur in that ash caused deep red and purple toning to form on many of the coins. The coin below, an aureus of Emperor Vespasian from the Colosseo Collection, is one of the most vibrantly-toned in the hoard. The reverse of the coin features spectacular blue toning, which represents the next progression beyond purple on the gold toning color spectrum (orange-red-purple-blue). Purple toning itself is very rare, but blue toning is almost unheard of on gold.
More Helpful Links
"Do gold coins tone?" - CoinTalk forum thread
"What is Coin Toning?" - CoinNews.net article
"How to Tell the Difference Between Artificially & Naturally Toned Coins" - Silver Dollar Scoop article
Colosseo Collection - A stunning gallery of Ancient Greek and Roman coins, with valuable background information on each coin.