What Causes The Rainbow Color On Silver Coins?

Until you become familiar with toning, be aware that there are many folks "producing" toning to sell specifically to uneducated buyers.

Check out coins found in NGC, PCGS, and ANACS to get an idea of what is and isn't natural toning.

Never seen an environmentally damaged (toned) coin that wouldn't benefit from a quick dip!

Now would you like to know what causes the colors?

It's called thin-film interference. Light passing through the oxidation film on the surface of the coin reflects from both the top surface of the oxidation and from the back surface of the oxidation. These two reflections interfere with each other resulting in a cancellation of some of the frequencies in the light. Which frequencies are canceled is a function determined by the thickness of the oxidation film the light passes through. Different thicknesses mean different frequencies and your eye sees this as different colors.

As for "what causes it", chemically, the answer is "corrosion from atmospheric sulfur". Silver does not oxidize under normal atmospheric conditions, but pretty much any airborne form of sulfur - be it sulfur dioxide, hydrogen sulfide or organosulfur compounds as well as liquid or dissolved sulfur such as fingerprint oil, onion juice or machine oil - will chemically react with the silver metal of the coin to form silver sulfide.

Bulk silver sulfide is black. The colours that we call "toning" derive, as Conder states, from thin-film interference. Basically, when the layer of silver sulfide on the coin is very thin, light can shine through it. The thinnest films are reddish in colour, then greenish, then blue. After blue comes black, because the layer becomes too thick to be transparent.

This does mean that, if you have a coin with colours that are pleasing to you, protect it from further reaction with sulfur in the air - otherwise, the colours will change, and eventually turn black.

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Can't help that, there were already seven posted before I got here.

very informative

The comments of Conder and Sap taken together, give the whole picture exactly.

In the light of these comments, it must be remembered that beautiful rainbow toning is in a very delicate and unstable state between blast white and full dull gray.
To preserve the nice coloring it is essential that the coin is kept in an airtight capsule. If that is not the case now, then take action immediately.

A slabbed coin is somewhat protected in it's prison, but slabs are not airtight. Over the long term, the rainbow toning may still be at risk.