You have a good question.
The words "silver fillings" as we have been informed throughout the years by a lot of dentists, is unfortunately a misnomer. It makes it a little confusing, doesn't it?
The exact metal composition in a "silver filling" varies, but it is approximately 50% mercury, 30% silver, and the remaining amount is a mixture of other metals like copper, tin, and zinc. Note that some of these dental amalgams can also have a much higher proportion of copper, so that the amount of silver for these types could be even less than 30%. I hadn't heard if these dental amalgams contain nickel, but it is hard to say exactly what may have been placed in your teeth. You can read what was discussed below in this dental healthboard concerning dental amalgams by various people.
Note that if you have "silver fillings" that contain mercury, you have mercury in various other locations in your body (like a lot of us), as it spreads throughout. Mercury is extremely poisonous, much more than lead, especially if is methylated. Some of us believe that it is a serious health concern. Others believe that it is safe to have mercury in a dental amalgam in your teeth. You'll have to read up on it from many sources.