I found this weird looking tan/brown looking rock somewhere in the mountains in Montana and noticed it had this weird spiral hole in it a decided to break it open and the inside was black and dark brown
as well as powdered almost like mushroom spores
I think you found a concretion filled with iron oxides. The spiral 'opening' looks to be what's left of a gastropod shell and fossil fragments like that often act as the trigger for mineral precipitation or the nucleus around which concretions form.
Could you turn the pieces over and take a photo of the outside of the rock like shape? It really looks to me so far is that the easiest solution is the most likely. It is a mushroom as far as I can see. It is also too bad the "screw" part isn't in tact to clearly see.