Ancient Pottery Lid (?)

I bought this item at a yard sale. The professor that I bought it from said that it was some sort of lid to a storage container.It is made out of pottery,terracotta,or etc. At the top there is about a one inch round hole in it. It is glazed, and crazed (small cracks over the whole surface). On the bottom left side has some type of maybe either volcanic rock, or other mineral stuck to it. It's non-magnetic, but I`m not sure what this is.The back is unglazed. I know this is very old,but I'd just like some information,such as how old it is, and an estimated value of this item.