How can I re silver my fake silver (metal) Locket?

My boyfriend gave me a beautiful Locket for Christmas this past december. Its been 4 months now and i wear it every day. the back of it is almost completely rusted or brownish. I tryed clear nail polish and everything nothing is stopping it. What can be bought or made that will make my locket silver looking again?? It means alot to me so please help!!!!