Crab Orchard stone is actually a patented trade name referring to an unusually hard standstone quarried from the Cumberland Plateau area in mid-eastern Tennessee, specifically around the community of, you guessed it, Crab Orchard. Crab Orchard is just north of I-40 about 60 miles west of Knoxville, TN. Crab Orchard stone is characterized by the unusually high level of silica which makes it very hard. In fact, the high levels of silica sometimes cause some geologist to classify it as quartzite. It varies between tan, buff, gray, blue gray, and pink depending on the trace minerals in that particular sample. Sometimes it has colored stripes, swirls or ripples. It is a multi-million dollar industry in Cumberland county, TN and has been used in buildings from President Franklin Roosevelt's home in Hyde Park, Elvis Presley's pool area at Graceland, the Vice President's residence in Washington, D.C., Rockefeller center parking lots, Nintendo office building in Honolulu, and Detroit's United Auto Worker's H.Q. in Detroit just to name a few.
As far as cleaning it, a mild acid solution applied with a brush and immediately rinsed off with plenty of water might do the trick. A diamond surfaced grinding wheel in the hands of an experienced operator could take off a fraction of an inch of the soiled surface and expose fresh stone. A replacement for the broken edges can be obtained from the many rock and stone dealers in the Cumberland county, Crab Orchard area. Good luck.