Monk...The artifacts that paleojoe is finding look to be Mississippian and date from about 1300 BP to 400 BP.
The sites that they are found on frequently have an abundance of clam/mussel shells that made up a large part of their diet, these shells over a period of time tend to neutralize the soil acids and help to preserve the bone objects.
If the objects were worn, ( Beads or Hair Pins) or pieces used as tools such as awls and fish hooks, the polish from use or intentional, helps to preserve them even more, due to closing the pores of the bone. Sites that are well drained tend to have better bone preservation also. Some of the more polished pieces almost look like plastic.
Hope this helps.