kat, my first thought was the same as Viking; it looked like a lug end of a soapstone vessel. I can't tell from the photo if that is soapstone, but like the name implies, it will feel like a bar of soap, a somewhat greasy feel to it, usually not coarse or sandy feeling, but slick. Metates can be just a slab with a hollow worn in or they can be well fashioned. Like yourself, I have never seen one with lugs, but I don't know your stomping grounds. My first thought was what it looked like if it were found here. Here's a photo of 3 from RI, with close up of small one with lugs. Lugs came in different shapes. Of course your piece could be a shallow bowl of a different material too. Sandstone for instance. The third photo makes it look pretty shallow whatever it was.