I have what appears to be some kind of Thuringian/Escher type hone... I call it the "Ye Olde Black Slick Rock." jk

Yeah, I know what you mean. There's a seller on the BAY from somewhere on the U.K., if I remember right, that always has a whole mess of stones for sale; I haven't actually bought anything from the seller, but I wouldn't hesitate for a second, if for no other reason than I've often seen him/her flat out say in the title of the stone for sale that he or she doesn't know for sure what kind it is. If that seller is honest enough to be like, "I don't know what the hell it is, but it's smooth and works like a charm", I'll trust 'em.