Silver pieces tend to oxidise over time. This can also happen to gold plated items.
There are a few things you can do to maintain your jewellery looking fresh and clean:
Remove your jewellery before engaging in physical activities including exercise (gym classes, yoga, swimming), dish washing or other cleaning. Chemicals found in household cleaners and swimming pools can damage your jewellery, therefore do your best to remember to take your jewellery off before.
Perfumes or beauty products should be applied before putting your jewellery on.
It is best not to sleep with necklaces, earrings or gold plated items. This will avoid possible damage and extra de-colouring of the gold plate.
When storing your jewellery, please use the box or soft pouch provided, separating each item to protect them from scratches. Avoid contact with air, moisture and other pieces. Even a small plastic bag can help avoid oxidation.
To clean, there are a few options:
1- Use a silver bath liquid (I use a brand called Hagert, it is sold to clean cutlery). Dip the jewellery for less than one minute and it will come clean.
2- Use a soft cloth (or silver cleaning cloth with tarnish preventative)
3- Use a soft toothbrush and white tooth past (yes it works), brushing it in one direction or if you prefer, in a circular way.
4- You can also use bicarb soda and lemon or white vinegar. For a matt look, use a piece of a kitchen green harsh sponge with soap and brush it in the direction you choose.