That green banned slate does not knap. It's grainy (though finely grained) sedimentary rock, and sometimes, ancient pieces show some chipping, but I believe that was to help get to the desired shape before any pecking and grinding and polishing... I have seen many fine newly made pieces from slate though, so there is a small market for stuff like that
It depends where you found it, near a glacial morraine and it could be natural. South of the morraine and it was probably carried in to be worked and then discarded.
It looks to have been polished a bit, probably to check the banding. How big is it?