I use a mixture of metals in my designs.
The sterling silver jewellery and other metals such as brass and copper which are used to embellish the silver are all natural materials and they will tarnish. This is a totally natural process of oxidisation.
All the jewellery can simply be cleaned with a jewellery cloth that will be available from most jewellers. The silver cloth will clean all the metals.
For more stubborn tarnishing, I use "Silvo" wadding which is available from most big supermarkets.
Simply follow all the instructions provided on any of the cleaning products you might acquire.