What can cause pocketing in the gums?

My sister is 81. She has pockets in her gums. Her complaint for months has been that there is something like a chain (small beads with a cross on it maybe)that is stuck in her gums. She says she has seen the chain, but it is wound tight and she can't get it out. She is afraid of swallowing it some night when she is sleeping. She gets obsessive with trying to get it out. She went to the emergency room one time and they took x-rays, but found nothing. She says she wants to get it out to prove to everyone that she is not "crazy", and that it is actually there. She lives alone. She seems to otherwise be very rational about things. She is lonely, however, and does not have a car right now to even get out to a drug store, and has to depend on favors from others. Something may be wrong with her gums because of the pockets. A dentist did not see anything either, by the way. Could this be the start of dementia? Or could there actually be something in the mouth like at the base of the teeth that could come out in what might look like beads? An old infection, maybe???