This article emphasizes the difference between artificially toned coins and naturally toned coins so as to make it easier for the individual to buy one for himself.
It describes the coloration that results from different chemical reactions that happen on the coin surface. It ranges from jet black to a multi-colored rainbow. When you go exploring the market, nicely toned coins ask for hefty premiums and often sold for twice, thrice and sometimes even ten times the price of the same untoned coin.
Also, keep in mind that silver is the most reactive metal that tends to react with sulfur to create a rainbow toning. Copper is more reactive than sulfur. When copper is toned, it darkens over time ranging from red mint luster to brown patina. A major portion of pre 20th century bronze or copper coins bear a dark brown color. If a coin has a red mint luster, then it has been dipped, restored or coated in shellac or lacquer. These are designated as RD grade; brown ones are BN and red brown as RB.
Artificially Toned Coins
Apart from the toning propensity of various metals, the coloration will help you ascertain if you are looking at an AT or NT coin. Many AT coins have extreme coloration. The deep blue or purple color, particularly when mixed with abrupt color transitions is quite a strong sign of artificial toning. When you tone a 1991 American Silver Eagle which is so pure silver dollar in the first place, dramatic toning is very common. But when you artificially tone it, it gives away a deep aquamarine color and a patch of violet or orange in the center of the coin. That deep color contrast signifies rapid toning which is a result of artificial toning. When you artificially tone a Morgan dollar, you will see many angular swatches and an abrupt change in color. It may look a lot like neon colors. So, be wary of buying such coins or you will end up paying a fortune for these.
Naturally toned coins
Now when you compare naturally toned coins with artificially toned coins, the dramatic hue persists, but it is more of an evenly balanced pastel and not neon palette. There is no significant color transition and the toning immensely blends from yellow to gold, gold to red, a touch of purple and blue on the boundaries. Toning only takes place on the coin surfaces which is in direct contact with sulfur coated fabric, which is why sometimes you will find one surface of coin toned and other being not.