Most types of chert, agate, jasper, obsidian, dacite, some petrified wood, some rhyolite, some quartzites, fine grained basalt, and a slew of other wierd stuff will knap. Some of the cherts , jaspers, and agates may need heat treating to work well. From a quick google search, looks like Idaho has a good bit of obsidian, rhyolite, and chert.