Perfectly smooth and shiny would work.

[meaning, leaving them unstamped for the moment. she doesn't exactly offer to pass the egg over even as he examines it.]

Otherwise, I'll enchant it with a design based on the world, or something. I can claim it's a rare type of gold from a different dimension. If they're going to accept it, they'll fall for a convenient claim like that. And then I never have to worry about buying what I want ever again.