That's a slate preform for a bannerstone. Rough shaped, and hole drilling started with a cane drill. Here's what it would have looked like finished.
Here's a pic of a banner I made that I just started drilling. You can see how the core drill starts drilling, the same as on your piece. Nice find. Bannerstones are rare. Gary
1st - I noticed this was your very first post - so, Welcome Aboard Arrowhead! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... and selecting location information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).