FWIW : Looks to me like a quarry blank of tabular (bedded) chert. About the right size and thickness.
You can't put a name on it until you have determined whether or not other pieces of the same chert can be picked up where it was found. If so, probably natural. If not, how far away from the find spot is the quarry
/weathering outcropping ? That's the determining issue re. whether it's an artifact (showing human modification of any sort) or not.