I'm not sure if this is an artifact, but it doesn't seem like just a natural rock and was different than all the other rock in the area.
Could it be a scraper or some other tool?
Looks like a piece of flint. But not really worked much, gun flints have that square shape but too big for that,but could of been what they were trying too make
You can definitely see flaking in pictures 3 and 4. I find a lot of Flake Tools that look just like one. Uniface and with at least one worked edge on them and sometimes some very fine edge work is done on these Tools.
I believe it's an artifact. Maybe just a practice piece, but more likely a quick knapped tool used for what was going on at the time at that site then tossed aside.