Prairie du Chien Chert (page is materials M-Z, so scroll down a bit for PDC entry)
Part 2: variety 2
The second variety of PDC is not as easily distinguished. It occasionally has a banding or grain, but this also occurs in Red River Chert. Because the distribution of these two materials does not overlap to a significant degree, they may still be distinguished based on the location of the site. The banding in this variety of PDC is contorted, somewhat like wood grain in a knot.
It’s not the easiest thing to see because of the color, but the flake in the second picture has a few vugs (little cavities in the rock) that are full of little quartz crystals. One is visible as a grey patch with some lighter spots on the right side of the flake, midway down the edge.