I managed to snag this one for virtually nothing over the weekend. <£10 shipped.
8x2 inch black stone in box. Natural.
Attachment 174388
Attachment 174389
Attachment 174390
I'm not sure what to expect with this one. It might be a slate, Or a thuri. It doesn't look like an Arkie.
Anyone got a better guess, Or want to speculate :D
sharpening stone Natural Stone | eBay