Gold or silver plated jewellery is made of a
base metal (such as copper or brass) with a layer of gold or silver applied to
the surface of the metal.  Acid from your
skin or from perfume/lotions can react with the base metals in your jewels to
produce a green oxidation on your skin.
This is not dangerous and can be washed away.  If you continue to have problems, try
applying a thin coat of nail varnish to the inside of your rings or the back of
your necklaces.