Wow Tim and Sweeney; these things are coming out of the woodwork. I bought mine on Ebay last year.

I think the gray stone on mine is a Thuringian. It's silky smooth and has the look and feel of a Thuringian rather than a slate.
I bought mine with the same idea, replicating it. Given it would probably be impossible to find a quality natural stone in the same dimensions let alone enough of them to make and sell, I thought maybe a Spyderco UF? I don't own one, but aren't they thin and fairly narrow?
The backside of the leather on mine has the Russian diamond grid pattern. Does yours?
Chris L