"pretty exclusive" . . . Do any dug Peltiers without AV show it? I think only a chemical analysis would convince me it's any different than the similar "abalone", "metallic sheen", or "irridesence", or whatever you want to call it, that I've seen on other dug marbles. It's definitely not worth arguing about though, that's for sure. Here's a variety ranging from "sheen" to "metal"
Sorry for the crappy pic.