Anyway - there is a real banded stone. This is it:
Time travel sine qua non?
I went hunting for it with my children one day while I was writing the story. I found it in one of the most magical places I know in Vermont, the wooded trails at Shelburne Farms. I'll admit that I haven't tested it for time traveling purposes. Of course, that would be pointless since it only works when you also have the correct bicycle and handlebar bag in which to place it. And those are not available to me. Still, it is a talisman. Maybe someday I'll pass it on to a specially interested reader...