Jay man, I am going to step out here a bit....knappable rock is formed from several events including volcanic eruptions and compressed vegetation and I am certain other events that occurred a long time ago. The different types of knappable rock have names given them by the manner they were formed. If you do a search or study of your area for "rock formations", then you may find a belt or vein of knappable rocks and were they formed. To give specific info it could take a while....
Get a piece of bottle glass bottom and strike it with a rock or copper bopper on its edge. The break you should see is a conical fracture. That type of break or fracture is what you want in knappable rock when you sample it. If it crumbles or crushes it is not useable. Some rock will break or fracture better than other so practice and knowledge is key.
Hope this helps and answers your question...Good Luck