All jewellery tarnishes naturally over time and requires the occasional cleaning.
I have found that storing my jewellery in plastic bags with an interlocking seal and in a jewellery box when not in use slows the tarnishing process.
Ocassionaly (when they stop sparkling) I wash my jewellery in a very mild warm bath, rinse with warm water and dry with a clean soft cloth, an old nappy or flannelette cloth is perfect.
I also find using a soft bristle toothbrush on my jewellery can bring more sparkle, BUT it must be soft bristles.
For my Sterling Silver and Gold Jewellery I polish carefully with a Silver and Jewel Cleaning Cloth.