I can tell you all of Silver plated, stainless steel , nickel plate, brass plate don't rust normal,,sweet iron is easy to rust(but if the plating layer is good,it is not easy to rust too). plating is the most important thing to rust.
what synthetic crystal you are taling about? do you mean buckle with stone? it is also need to have a good plating then it is not easy to rust, if not. can you send me picture so I can tell you more information.