This is completely off the wall, since I know basically nothing about Woodland-Mississipian era stuff except the little that's been absorbed from casual exposure.
That said, Mesoamerican cultures like the Aztecs (as I recall) did do some
mighty fancy knapping (stuff like this only bigger, and even a lot more complicated & difficult). Show-off stuff.
Now. Hopewell mirrors a lot of their ideas and techniques in kind of a distant parallel.
So, whatever way it happened, there was clearly communication going on between Mexico and the "O" states here. So you would just about
expect to find evidence of this. Which your father-in-law apparently did. (Or whoever he got it from did). Assuming that was 50 years ago, how many fakers with this skill level would have troubled to (or even thought about) burying it for long enough to pick up the kind of mineral deposits from the soil it shows ?
Nobody knows. But, considering it as an abstract proposition (how else could you approach it ?), it's certainly a plausible item, and possibly a significant one.
My admittedly
--deleted-- two cents' worth
