I can see why one might suspect that, but it is important to remember that ironstone is a sedimentary rock, formed by sediments being put down in layers. These layers often have bands that contrast in color and and density, causing different rates of erosion over time. A denser band of sediment would then project from the softer surrounding minerals. The layers are most often wavy, but being as straight as in your sample isn't unusual, given how it is formed. That being said, I've learned to never say never!I'm thinking I'm getting closer to the origin of this stone. The straight line still doesn't seem natural.
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I placed the straight edge on it just as a reference. I also pointed to the other line that if continued on would be parallel. The other stones I found there were relitvely brittle. My original thought of it being slag was based on the fact that it was just dumped there and used as fill. If it was useful in any way they wouldn't have wasted energy to dispose of it. Could this possibly be the byproduct of a stone cutting operation. I know there can't be many around here but this kind of makes sense. When cutting tiles on my wet saw I wind up with a layer of sludge that if I let dry would look just like the tile just more brittle. Could this be the byproduct of cutting brownstone? Stone dust as opposed to saw dust?