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Tarnishing of silver, which is typically a blue/purple/black colour, is triggered by its reaction with elements such as sulphur, moisture, oxygen and chlorides.
Oxygen and Sulphur compounds in the atmosphere, such as sulphur dioxide, hydrogen sulphide gases and organic vapours, all contribute to the tarnishing process, which is also accelerated by other factors.
In addition to these naturally occurring accelerants, jewellery manufacturers themselves may inadvertently contribute to the tarnishing process.