I take the same view as Mark from Peep Show.
"How could you possibly make one of these except with some kind of magic?"
"...in a factory? With glass?"
Yeah, but according to the OP, scientists have dated them to be actually thousands of years old. I could see if they hadn't been verified to actually be that old, but they have, so your(and Mark from peep show) theory holds no water whatsoever.
This is definitely interesting OP, I never knew they were actually real. Perhaps they were done by shamans/witch doctors/whatever they were considered before they became the official tribe witch doctor. Like, it was their rite of initiation to become so?