Those all appear to be quartzite, which isn't bad for a hard hammerstone. Ideally you want a soft hammerstone for flaking purposes and those tend to be a softer material than quartzite. The hard hammerstones are good for initial reduction but tend to produce thicker bulbs of percussion. Soft hammerstones produce less thick bulbs of percssion and are better for final flaking. The ones we find here in Arkansas that are soft may typically be cobbles of a hard sandstone. Try one and see how it goes. If your hammerstone isn't wearing some and producing flat surfaces after moderate use, you may seek some softer stones. Like I said, try some of these and see. You can get good results with most stuff as long as it is softer than the stone you are whacking, so don't worry much and see what you can do.
