Sterling silver will tarnish due to interaction with sulphides in the air. The tarnish is golden in hue when it begins, but will eventually progress to black. Higher sulphide levels are associated with humidity and air pollution; the more humid or polluted your environment, the more quickly your silver will tarnish.
• Dip the piece into a small bowl that has been filled with lemon juice (or vinegar). If tarnish persists, add some table salt. Rinse in clear water and dry with a soft cloth.
When your jewellery has stones in it, particularly “organic” gems, do take care! I use only a soft polishing cloth to clean jewellery incorporating organic gems.