I live where that is the primary knappable rock. I chip a lot of that Texas River Gravel. Some if it is premium, but what is available around here is tough as nails. I have to slice it and then cook it before it is reasonably chippable, then a lot of it is very good, and the colors are super. Of course that involves using a rock saw. Most people are not equipped for that.
If you cook those rocks whole, there will probably not be a one that survives. They fracture to little pieces if they have much thickness when heat is applied.
If you want to heat them, there are several ways to do it, but the easiest is using a $25 turkey roaster. If you are interested, I can tell you how to do it, but remember that they need to be thinned by spalling or sawing before heat is applied, else you wind up with lots of little fragile pieces of sharp rock that is worthless.
WA