Atlas sells a grey colored "pearlescent" rod. This would make a nice looking ring. Obviously, it wouldn't be as shiny as metal, but as far as alternatives go, this would be my choice.
That is just acrylic rod. That will not be near as shiny as metal.
That is also the same thing as the "pearlescent" rod that yahosh is talking about from Atlas, just in a different color. It will finish nicely but surely won't shine like metal will. But on the plus side, it comes in every color in the rainbow. Just for reference, Chris Hightower sells it too so you don't HAVE to go to Atlas to buy it.