Some info o tarnish and silver:

"What is Silver Tarnishing and What Causes It?
Tarnishing, also called toning, is an effect on the outermost layer of a metal that causes the metal to become discolored. Like iron rusts when exposed to oxygen, silver can tarnish when exposed to the air due to a reaction between silver and hydrogen sulfide in the air. Also, touching silver products with your hands may cause tarnishing over time due to the oils from your skin.

Some products tarnish quicker than others. For example, because of the finish on the American Silver Eagles [coins], they tend to tarnish slower than traditional bullion coins. On the other hand, the Somalian Elephant coins tend to tarnish quicker due to their storage tubes being made of PVC as opposed to the traditional plastic tubes that most other coins come in. The PVC traps moisture and releases slightly acidic gases which can contribute to the toning/tarnishing of coins / silver."