One of the first terms a beginning numismatist will encounter is toning. In essence, toning is a fancy way of saying tarnish; both words refer to metal reacting with its environment. Just as a silver plate can change colors over time, a coin can (and often will) do the same. While tarnish is often considered a negative term where silverware and such are concerned, toning can be either positive or negative for coins.
Copper and silver coins are by far the most likely to tone. In particular, these two metals are especially reactive to sulfur. A mild amount of exposure to sulfur will result in beautiful blue, purple and magenta hues. Over-exposure to sulfur, meanwhile, will turn a silver coin black. Coincidentally, most 19th and 20th century coin bags, roll wrappers and albums contained some amount of sulfur. This resulted in some spectacularly toned coins, including some with wild rainbow patterns. These pieces often trade for dramatic premiums.

Silver and copper also react with ordinary naturally-occurring substances in the air. Unless it is stored in an inert holder or case, a silver or copper coin will almost certainly acquire some toning over the course of 150+ years. This is why virtually every coin from the late 18th and mid-19th century displays some amount of toning. In fact, it’s a virtual guarantee that any pure white silver coin or bright red copper coin from this era was “dipped” in a chemical solution to remove the toning.

Gold and nickel are much less likely to tone, as they are more stable metals. However, American gold coins can still display reddish and purplish toning due to their copper alloy. Since 24K carat gold is extremely soft, the US Mint mixed a small amount of copper into their gold coin alloy. This made the coins much more durable—and resulted in some US gold coins turning attractive shades of red and magenta.

Toning can be a double-edged sword when it comes to affecting a coin’s grade, desirability and value. Attractive and eye-appealing toning can and often will help a coin’s grade. In addition, some coins command tremendous premiums for their toning alone. A common MS 65 Morgan silver dollar is worth $150-175, but a superbly toned coin can sell for well over $1000. On the flip side, ugly toning can negatively impact a coin’s grade and market value. Dark, brown, black and splotchy toning is considered highly detrimental.

To summarize, toning is a common phenomenon in United States coinage, especially for copper and silver coins. It is considered perfectly normal, but extreme cases of toning can either help or hurt a coin’s value. Vibrant and color toning is considered tremendously desirable, but unattractive toning will almost always diminish a coin’s value.