Coin Toning 101: The Differences between Naturally and Artificially Toned Coins
Pop quiz time! Take a look at the picture below. Of the four Morgan silver dollars, which would you guess are naturally toned and which are artificially toned?
Finished thinking? NGC graded all four as naturally toned. The color on these coins spans the entire spectrum of the rainbow, but the NGC grading experts determined that the toning emerged naturally over time rather than being artificially created. But if coins that span such a wide range of hues and colors all fall under the banner of natural toning, how can a collector distinguish artificially toned coins? This article will reveal the main indicators used by experienced coin collectors to identify artificial toning, helping any toned coins enthusiast to tell the difference between artificially toned and naturally toned coins. It's important knowledge to have, as attractive natural toning can make a coin worth exponentially more, while artificial toning can completely ruin a coin's collectible value.
The Chemistry of Toning
Any article on toning is best started with a quick chemistry lesson on what actually causes toning. At its basic level, toning is the result of a chemical reaction between the metal surface of a coin and an atmospheric element (most often sulfur or oxygen). As that reaction occurs, a compound forms on the surface of the coin, gradually changing the color of the metal. The color change is a gradual process, and typically takes years to fully develop - most vibrant naturally toned coins acquired their color over decades of storage in conditions that were conducive to toning. The environments most conducive to toning are often high-humidity with sulfur present (often in paper envelopes or bank bags), but that is only a general rule - toning can and will happen anytime and anywhere, unless a coin is hermetically sealed from all contaminants.
Some individuals try to recreate those toning-friendly conditions to induce toning within a shorter period of time. These "coin doctors," as they're termed in the hobby, expose their coins to sulfur and other elements in humid, high heat conditions - trying to artificially speed up the toning process to produce vividly toned coins. The techniques they use range from sophisticated to crude - there are plenty of online articles promising great results by sticking the coin in a potato and baking it, storing the coin next to hardboiled eggs, and other equally ridiculous techniques. All methods of artificially inducing toning are condemned by serious coin collectors, and artificially toned coins are considered "problem" coins, just as if they were damaged, cleaned, or polished. A coin that has been artificially toned loses its collectible value, and will generally sell for only the bullion value of the metal. Third-party coin grading services refuse to assign artificially toned coins a specific numeric grade, instead grading them as "details" (i.e. "uncirculated details") and noting on the slab label that the coins have been artificially toned.
Toning Propensity of Metals
In order to identify an unslabbed or "raw" coin as artificially toned, it's important to understand the toning characteristics of the coin's metal - copper coins tone differently than silver coins, and silver coins differently than gold coins. Of the metals most often used to produce coins, copper is the most chemically reactive and therefore the most prone to toning. When first minted, copper coins have a bright red color. Over time, the copper oxidizes and the coin's color darkens to brown. How long that process is up for discussion, but most 19th century coins and many 20th century coins have now toned to a brown color. Copper coins with their original red mint color (marked "RD" on slabs) are consequently worth a premium over brown (marked "BN") or mixed red/brown coins ("RB"). Copper can also acquire "rainbow" multicolored toning, but it is much less likely. 
Any raw copper or bronze (copper/tin alloy) coin with dramatic rainbow toning should be evaluated with slightly more skepticism, though it's certainly possible for the coin to be naturally toned.
Second only to copper in terms of toning potential is silver. Like copper, silver usually oxidizes over time towards a grey color. Silver frequently reacts with sulfur to form rainbow toning - the Morgan dollars at the top of the article are examples of that process. Nickel coins are less prone to toning than either copper or silver. Nickel by itself is a fairly unreactive metal, and generally only oxidizes to a deeper grey color over time. But when alloyed with copper, as with most modern U.S. coins (the nickel itself is only 25% nickel and 75% copper), the metal becomes slightly more reactive. Rainbow toning is not uncommon on copper-nickel coins, though it should be viewed with more skepticism than rainbow toned silver or copper coins. Finally, gold and platinum are very unreactive metals. Gold oxidizes to an orange color over time, and very rarely red or purple. Platinum hardly tones at all. Rainbow toning is almost unheard of on either metal. 
Artificially Toned Coins
Setting aside toning propensity, the color patterns of artificially toned coins can be a strong indicator to collectors who know how to spot the signs. As a general rule, artificially toned coins tend to have more dramatic toning - coin doctoring is not a subtle art. Deep blues, purples, and reds are common on artificially toned coins, often with no transitions between colors. A naturally toned coin will tend to have gradual transitions between colors, along the normal color spectrum- green to yellow, yellow to pink, pink to red, red to violet, violet to blue, and blue to green, for example. A coin with entirely separate swatches of color and no transitions should be viewed with suspicion. Taking a look at the coins above, note the deep blues and reds and how the colors abruptly shift without gradual blending.
Naturally Toned Coins
Now take a look again at the Morgan dollars from the beginning of the article. There are still blues and purples, but the colors are lighter. More significantly, the colors blend into each other smoothly over the surface of the coin. It's also worth noting that each of the coins has visible luster and is in mint state condition, whereas the artificially toned Morgan dollar and the Franklin half dollar from the previous section show no signs of luster. It's relatively less common for circulated coins to have vibrantly colored toning, so that's another warning sign. The type of toning seen on circulated coins is more often a darkening of color from oxidization, not rainbow toning.
