When talking about stink rock and you say a sewer or petroleum smell, is that similar to a gun powder smell? I ask as much of what I am finding and hoping is knapping material, smells like gun powder after taking a wack at it. It does produce sparks on my metal detector found file. Problem to me is sometimes it has a fine gain texture when dry, and smooth to slippery when wet. Seems to be the same whether it is green, brown, black yet grey inside, or even the red rocks. This stuff is very hard to break with another large rock and has shot out at my shin like a bullet, knocking me down and leaving not a chip on the stone. I am in central MN and I know granite very well, but I do not know chert about flint. Any Ideas??