Another type to be aware of is the 'hard-to-handle' rock. The kind that play and lay 'dirty'. They never seem to be happy where they are put. These are mean stones that not only pinch and crush and give you what Kevin Gardner refers to as a 'granite kiss', but have an agenda and want to bite your whole leg off. They are the temperamental ones which you would do best just to avoid. In thinking with my hands, is it ironic to describe these easily irritated stones as 'touchy'? Perhaps they are related to 'erratics'? Basically everything about them is edgy and angular. Their shape seems totally unadaptable. Their hardness only makes them more hard to handle.
While most useful walling material almost 'speaks' to you (if you are patient enough and take time to listen) these rocks don't seem to have anything to say. If you have to, place these rocks very carefully in your wall but remember you don't have to use every rock that you have in your stock pile.