Tough to say from a distance like this, but it does remind me of a "White Hard Arkansas" I bought in Needles, California, 20 years ago. Traditionally, the White Hard Arkansas were and are a milky white in color, but this one from a hardware store in Needles had a vein of yellow-brown in it, almost rust color, that resembles the center section of the stone in your photo. I suppose it always depends on the area of the quarry it comes from. How big is yours, and where did you get it?
Sam