Tarnish of Jewelry is also possible due to surroundings, we live in. In most of the areas in Punjab, people face stagnant water in their locality. It can be water after rainfall or water from sewage or from any other reason. Scientifically speaking, their are number of gases in the sewage water, which when comes in contact of silver, can turn silver into grayish black color.
This is the reason, why anklets usually turns black mostly of the ladies who live in such locality.
However the silver can be washed at home with different ways.
Precaution: Remove your Jewelry while dealing with stagnant water. After coming from stagnant water, immediately wash your feet and jewelry with clean water.