They're anodized aluminum.

The anodizing has to be stripped off, grille polished and straightened and then re-anodized. Anobrite or King of Trim can do this for you but it's spendy.

Personally, I did it the "backyard way" on my '63 330's grille and stripped the anodizing with Easy-Off (anything with Lye will strip anodizing), polished it and left it in bare aluminum.

How long will it retain it's luster? Dunno, but it's garage kept 99.9% of the time.

Keep in mind this is after hours of tedious straightening, wet sanding, polishing with Mothers and tedious masking and painting.