It is a matter of choice for a collector as to whether they collect mint perfect coins, dirty coins or those that show obvious toning. Toning is something that occurs when the metal of a coin reacts with other chemicals, most notably sulphur, causing a film to form on the surface of coins, a film which exhibits a range of colours. There are many types of toning, but one of the most spectacular is that of rainbow toning.
Rainbow toning is basically a film that produces a range of colours across the spectrum upon the surface of coin. This is something which happens frequently on silver coins and also on nickel rich coins.
If a collector was looking to artificially rainbow tone their coin then there are a number of chemical compounds that can quickly bring about the desired look. The chemical involved are often a combination of sulphuric acid and other sulphides, the sulphur being the main cause of toning on coins. Those coin dealers and collectors that do tone their coins almost always have their own preferred compound to achieve their desired results. This form of rainbow toning though is often easier to discern as the results are often too perfect, and really do resemble a rainbow.
A slightly more haphazard method of rainbow toning was discovered by accident. It was noted that the use of some mint bags, especially those with high PVC concentrations would over time cause a rainbow tone to build up on the coin stored in them. A slower process, the rainbow tone that results is not so perfect, but can be replicated in type time after time.
It should be of course noted that the fact that a coin has an extremely impressive rainbow tone upon it doesn't make it more valuable or any more collectible. Many collectors are put off because of the large number of faked toned coins on the market, and as a result stick to non-toned coins instead. This is clearly seen when looking through any of the Internet auction sites; where day after day rainbow toned coins go unsold.
It is not always possible for an amateur coin collector to tell whether the rainbow toning on their coin is naturally made or artificially enhanced. It is something that even reputable grading services struggle with. Collectors therefore need to be aware how rainbow toning occurs, and it may help them to be on guard in the future.