In short, silver reacts with sulphur oxides present in the air (e.g. hydrogen sulphide) and a black substance is created called silver sulphide.

In order for silver to be strong enough to work with and to have longevity as a piece of jewellery, copper is added in its manufacturing process.

Sterling silver has a purity of 92.5% silver (hence the 925 stamp in the hallmark).  This means that 7.5% of the alloy is copper and other metals like zinc.