I'm a little confused by this. I boiled water for 60 minutes in mine, during that time I saw the brown-grey color others have described. Upon cooling/dumping the water, however, the color changed to a quasi-whitish "powdery" looking coating. It's not powder, as it doesn't rub off, but it's not the dark color it was while boiling.
I assume it's aluminum oxide, but it doesn't really match other's descriptions.