It dosen't look very promising. You'd be better off with bottle glass, porceline (johnstone), TV tubes or floor tiles. That stuff doesn't show any sign of what is called a conchoidial fracture (breaking like glass).
I am pretty sure that the stuff you showed is worse than all the materials I mentioned above. You can take any bottle, slip in the largest bolt that will fit in the neck and shake it up and down until the bottom breaks out. From there, knapp the bottom. If you have a place near you that makes glass table tops or such, beg some strips off them. Floor tile from the big chains often is good. The slicker the better but judge it by the bottom of the tile, not the top.
Good luck, but that railroad ballast you showed is not at all promising.