And if it has some part of the skin (raw rust colored spots) intact that would be interesting too. From the picture it is also hard to tell what is stone spots and what is swarf from sharpening. A picture of the stone just wet and without any swarf is best. And the cross section.... A lot of factors in play here, sir.
This was the ebay auction and he had different pictures. I know nothing about these but based on size and color and the seller stated it felt like glass (surface) I was thinking it was maybe an old world, high quality stone.