Is this species of stone wall-worthy
When you say brittle is it flaky?
It looks like shale if you hit it or drop it on a hard surface it breaks up that would be shale, I have used it because my grampa had a hill that was shale and we used pry bars to break chunks out it works good for walls because it breaks and you can chip high spots off so they fit better if you don’t get parallel top and bottoms most of ours was like layers but different thickness. But it made a nice wall over a couple of summers (I think grampa was just keeping me and my brother busy.