Attached are picture of something I found in the badlands of north west North Dakota.
I had stopped at a scenic lookout for a couple pictures and to look around. I noticed, 150ft or so, down the slope a bit there was some erosion (sinkhole-ish looking). The hole was not deep and the sides were sloped enough that the risk was acceptable to enter. Inside i saw an abnormality in the color scheme of the sandy tan wall face (I was looking at the interior side of rock pictured). I dug it out of the sandy clay like soft soil, likely not undisturbed being so close to road, I would think it has been pretty disturbed during its construction. I have no idea what it is but its look flows like a hardened sediment and its "skin" is very scaley and rough. Also, the open side looks to me like it has some crystal mineral structure and the coloration may be the presence of metallic minerals? Which may explain why it seems a bit heavier than it should be.