Sterling silver is also known as 925 silver, this is because it contains 92.5% solid silver purity, the other 7.5% is made up of other alloys which are added to make the silver harder as pure silver is too soft to work with, the other alloys can be made up of zinc, copper or nickel.
925 Sterling Silver will always tarnish over time. The reaction to the air, fingerprint marks, cosmetics, detergents or even our sweat can cause blackening or yellowing of the silver. There is no need to panic this can easily be remedied.