why do some coins have rainbow toning? is there a particular reason or is it pure chance? Also, is it inevitable for coins to tone or is it possible to protect it so that it won't tone?
Two main reasons are the air and sulfur. Sulfur was used by the US Mint on the 1K coin silver dollar bags which prevented deterioration of the fabric. Any coin which came into contact with this material was going to tone somewhat and with the US Mint keeping these bags lockd up for many, many years there were some surprises to be had for sure... High sulfur content envelopes will cause this alchemic transformation as well. Coin envelops (go figure ) and the philosophers stone are notorious for speeding this process along. There are several excellent reads on the topic. Bone
Toning on coins can be prevented, but it's not easy. Circumventing nature is never easy - and it's the nature of coins to tone. But what causes a coin to tone in a particular way is dependant on many things. As has already been mentioned, the container the coin is stored in plays an important part. But so does the area of the country you live in - relative humidity & temperature variations. Then the way you heat your house matters too - natural gas, propane, heating oil, electric heat - each of them will result in different effects when it comes to toning. How you cool your home - is it central air or a swamp cooler ? One removes humidity and the other creates it. Smoking will affect them too. What's near your home matters too - is there a paper mill, a steel mill, any kind of manufacturing plant - in other words, anything that discharges chemicals into the air - all of these also have their own unique effects on coins toning. You see - the variations and the things that cause coins to tone in a particular way are infinite. But the thing that stops toning is singular - all you have to do is don't let the coin come into conatct with anything - even the air.
