A friend of mine in the business recently sent me this photo and asked me if I knew what it was (I didn't, but he did). It is a Lye stone. Lye stones were used to leach lye out of cinders. Lye was used to make soap.
This antique granite stone is about 36"x36 and is being used as a step. I think it would be a good looking inlay in a patio. If you are interested in learning more or might like to acquire it give me a call 617-797-6483 Gavin