For many years coin collectors have vigorously debated the definition of “natural” toning. In my opinion, natural toning is perhaps better described as “by chance” toning. Without intention, a coin is stored in such a way that it acquires coloration that some collectors find attractive. For example, United States silver dollars produced from 1878 to 1921 were typically placed by the minting facility in canvas bags containing 1000 coins. These coins were then shipped to banks for distribution. In many parts of the country silver dollars were not in great demand and so it was common for these coins to remain in the original bags for years. Thanks to this series of events, it is still possible to find coins from these bags that exhibit little or no surface oxidation. It was by chance that these coins were not distributed and used in commerce. Likewise, with other coins of this vintage it is entirely by chance that exposure to certain conditions provided a silver coin with a superb assortment of rainbow colors beginning at the edges and ending before reaching the still bright center of the coin. In my opinion, it is this truly “by chance” circumstance that appeals to many collectors of toned coins and makes these pieces special.
Once coin collectors got involved the matter of how and why certain coins managed to acquire attractive toning changed dramatically. I think it's fair to say that the majority of collectors from any period attempted to safely store and protect their valued coins using the best available practical means. As we now know, some of these methods resulted in the creation of beautifully toned coins. For the toning experts it is sometimes possible to determine that when a certain coin was placed in a certain storage device the coin had a brilliant appearing finish and the storage device was responsible for the toning. For lack of a better phrase, I'll call this, “unintentional” toning. There have even been examples where a minting facility has provided storage methods resulting in the toning of coins. I'm certain the Royal Canadian Mint didn't package certain of their collector sets knowing that if the coins were stored in the display cases they would soon acquire a deep gray or black oxidation.
Now that the numismatic community knows how chemicals in general and certain storage methods can predictably add or remove coloration, questions concerning “originality” and “natural” toning have become much more difficult to answer. For example, if I take a fully brilliant 1967 Canadian dollar and place it in one of the notorious RCM display boxes with the intention of having the coin eventually obtain a grayish color am I “artificially” toning the coin? I could argue that I'm doing nothing more that replicating the process that resulted in the “unintentional” toning.
The general interest and appreciation of beautifully toned coins is a fairly new trend. I can remember a time when the vast majority of collectors preferred coins with no oxidation. A common practice was to use liquid silver cleaner to remove any trace of toning, attractive or otherwise. Many beautifully toned coins were lost to the collecting community due to “dipping”.
Aside from the technical grade, the appearance of a coin can have a major influence on its desirability and value. For those who enjoy beautifully toned coins the color adds an additional dimension to the piece and gives it a distinctive if not unique character.
While I enjoy owning and examining beautifully toned coins I do so with the understanding that my fondness for these coins is highly subjective. What may be pretty to me may be ugly to you.
My experience has been that even after diligent examination I'm unable to always distinguish what might be termed natural toning from coloration that has been intentionally added to the coin. For this reason it would be impossible for me to sell a coin and state to the buyer that the colors that appear on the coin were encountered by chance and are thus natural.
My purpose in providing this long commentary is to suggest owning and enjoying beautifully toned coins should be based on personal taste and preference. If asked to pay a premium price for a nicely toned coin the buyer should do so with the understanding that this added value may not be recognized by any of the people who view the coin next-including the current seller.
It should be mentioned that in some cases coloration has been added to coins to hide or diminish the appearance of surface flaws. The most common hidden flaws have to do with cleaning and hairline scratches. Deep gray/black color provides an antique appearance and has fooled numerous collectors who thought such toning guaranteed originality.
My experience has been that the various professional grading services will refuse to grade and holder coins they think have been artificially toned or will note this condition on their containers. As with grading, the ability to deal with a coin's appearance in regard to toning apparently varies among the grading services. I think it would be presumptuous to conclude that the employees of all grading services are equally able to detect what the marketplace may currently deem artificial toning. As mentioned previously, this is often a highly subjective topic and has occasionally resulted in some passionate debates among recognized experts in the field.
I'm not sure I've accomplished much with this bit of writing if the best advice I can offer is to buy toned coins only if the origin of the color doesn't matter and may change in the future.