I have a couple of nice 18-inch necklaces, one 14K gold and the other sterling from a good designer. Both are fairly expensive pieces, and I wear them frequently. I use a jeweler's cloth to polish them occasionally even though they are _always_ shiny, never discolored. The cloth always turns black where I've rubbed the necklaces, no matter how long I clean them.
On the jewelry cleaning query. I found the best thing to use is
Fantastic spray cleaner by Dow chemicals. It is safe to use.
Unlike sudsy amonia it doesn’t leave a tendency to oxidize your
metal. It does remove the polishing compounds quite well.I found
in my expierience that it leaves your jewelry more sparkley