Do you know the difference between silver plating and sterling silver? Most fashion jewelry is silver plated. Unless it is specifically marked otherwise you should assume that your piece is plated.
Silver plating is a very thin layer of silver deposited on to a base metal such as copper, brass, or nickel. These jewelry pieces are more durable and generally less valuable then sterling silver jewelry pieces. If you wear the jewelry in water and don't care for it properly the plating will eventually rub off.
Sterling Silver is at a minimum 92.5% pure silver. It is valuable and sought after as it can easily be melted down with the pure silver extracted. Sterling silver will also always be marked as such with a 925 stamped in it. Sterling Silver will tarnish but can be cleaned off with a special cloth designed to remove the tarnish.
You can minimize tarnish on all silver pieces by storing them in an airtight zipper bag when you are not wearing the jewelry.
*Quick Tip: Never buy Sterling Silver from China and Mexico. Even though the pieces will be marked as .925 it is usually a fake.