Silver-looking Loonie

While my parents and I were cleaning the living room near the begining of September, we found a silver-looking loonie in a silver jewel box. My parents sait that they placed the loonie in the box in 1990. Now it's silver-looking and we all don't know why. Most likely has something to do with science.

We put two pennies in the jewel box and I am looking forward to opening the jewel box to see the pennies in ten years.