Alma 37 :5

"And now behold, if they are kept they must retain their brightness; yea, and they will retain their brightness; yea, and also shall all the plates which do contain that which is holy writ."

I like how it says that the plates will maintain their brightness.  I take this literally and figuratively.  The plates are all made of metal and I'm sure are preserved beautifully and remain shiny.  Figuratively these plates hold extreme brightness, light, and knowledge.  These scriptures will always have the light of the gospel of Christ within them and will never dim.