Hey folks, that's great, thanks so much for your input and ideas!
I'm no materials specialist, and I'm unsusre of the difference between stoneware and ...stone, but it sure feels like a stone to me. I guess maybe I can find a geologist friend. There is a local historian round here in a small library/museum thing in the town, it's not open very often but I'll try and pop by there with it, it's very intriguing.
I just love thinking about all the peope who might once have been playing with it, who, what and when
I've dug up a lot of stuff in my gardening history but this is up there on the very short list of particularly cool stuff!
Cheers