"Toning" or "discolouration" is, in a word, corrosion. Usually a very, very thin layer of corrosion, but corrosion none-the-less. Corrosion on coins is caused by exposure to the environment, usually some component of the atmosphere. If we're talking silver coins, then the atmospheric culprit is sulfur - usually in the form of sulfur dioxide or hydrogen sulfide.
Sulfur in the atmosphere can come from several sources. There is a small amount present in "natural" air. Living in a polluted city obviously has more. Kitchens where onions and similar foods are prepared cooked also create a large amount of sulfur in the air. Finally, sulfurous gases can leach out of various kinds of wood, paper and other "solid" objects over time. Natural Rubber, in particular, is very high in sulfur and gives off a lot of sulfurous gases as it slowly decomposes.
The vibrant colours of "toning" that coin collectors seem to appreciate are caused by thin film interference
, where the "thin film" in question is a layer of corrosion by-products sitting on the surface of the metal. The thinnest films cause red colour, then it moves down through the rainbow: yellow, orange, green blue. Once you get past blue, the corrosion layer becomes too thick for light to pass through it, and it becomes opaque - which we see as black. At this point, people usually start to call it "tarnish" rather than "toning" and it suddenly becomes undesirable.
Not all discolouration is caused by atmospheric effects, however. Black spots, often called "carbon spots", are caused by small droplets of water landing on a coin. If the drops are generated by someone coughing or sneezing, then it ain't water, it's saliva - which contains all kinds of enzymes and chemicals which can accelerate the formation of a corrosion patch where the droplet landed. Fingerprints, of course, are comprised of oil from your skin, and that oil is quite rich in sulfur compounds - which is why fingerprints turn black on coins.
Both "natural toning" and "artificial toning" are caused by chemistry, where the metal reacts with something in the atmosphere to create a corrosion by-product. Using the terms "natural toning" and "artificial toning" can be deceptive; if identical chemicals are used to create "artificial" toning, then there is logically no way to tell the difference. The coins, and the chemicals reacting with the coins, cannot measure or preserve the intent of the person causing the coins to be exposed to the chemicals. If my Grandma kept some coins in a yellow envelope for a few decades and they came out all lovely greenish-orange toned, that's considered "natural". But if I did it to some otherwise-identical coins with the specific intent of harvesting such colours in a couple of decades time, that would logically be "artificial" - yet the chemicals and circumstances are identical. The only difference is in the intent: my Grandma wasn't trying to create pretty-coloured coins, but I was.
When describing the more criminally-deceptive kinds of coin alteration, it is perhaps better to use "accelerated toning", rather than "artificial toning", because it more clearly describes the intent and effect: to attempt to reproduce the effect of years of natural aging, but to do so in hours or days, rather than years and decades. Accelerated toning does indeed require the use of chemicals not found in nature, and it is those chemicals and their by-products which can most easily be detected.
Hope this helps.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis