Its grey (or gray to you guys across the pond:-) with a lighter grey speckle. Found at local flea market covered in congealed oil.

I was actually a geologist before I was a tool guy, but it has been a long time since I was a mineralogy teaching assistant.  It looks like a siltstone with quartz sand grains, but it is hard to tell from a photo.  Can you see if it will scratch glass?  It would help if you could try to isolate the dark gray and light gray particles when scratching to see if they both do or just one.

Ok I will give it a try

I have tried the scratching glass test. The darker grey areas are scratched with the glass, and the pale grey areas (the small speckles) scratch the glass. Hope this is helpful.

It is hard to tell from the photos, but it is probably a very fine siltstone with quartz sand grains in it.  It looks like a sedimentary rock from the photos.  It seems like an odd choice for a sharpening stone.  A more homogeneous stone would probably work better.  Most of your rough sharpening will come from the side with the most grains.  I would use the side that is darker with fewer grains for finer honing.

might be related to the belgian blue stones...