Hi, I am new to the forum. I stumbled across you guys while looking for info on antique duck decoys. My Girl, Dee and I are "Treasure Hunters" of a slightly different sort. We comb thrift stores, garage sales and auctions. We live in the San Fransisco Bay Area where there are a lot of historical spots for Metal detecting, which I have been thinking of getting into. So when I came across the site, I decided to join. So Hi!
Since I do not yet Have a metal detector, I thought I would debut in this area where I might be able to contribute something .
Anyways, here goes.
the best way to tell if something is bone/ivory vs plastic is to take a sewing needle (held by pliers, forceps, etc.) and heat it with a lighter. Get it hot hot. Find an inconspicuous spot on your artifact and attempt to push the needle in. If it is any kind of resin the needle should go right in. If it is bone or ceramic, the worse you will have is a tiny little burn mark that can usually be rubbed out.
I would tend to agree that it probably is from the era of your other finds if it came from the same strata. However, because it was found in or near an active water way, it may be that it was deposited there with the other items that may have been displaced by flood, excavation or some other event.
Hope this helps.
Good luck,
Ursus.