Unlike the few problem-types we come across occasionally, (examples of which I described earlier this week) rocks are simply a pleasure to work with. Masonry restoration expert, and good friend Bobby Watt explains that the difference between a rock and a stone is that "a rock is simply an unemployed stone". It is safe to say all kinds of rocks will end up being very useful in your wall if they get half a chance. When you try to build with them, it's as if they actually 'want' to be part of your dry stone wall. They often have at least one or two nice faces. They are usually 'kind' faces too, the kind that are agreeable in terms of working together well with others in a wall.
Almost any rock will 'work' naturally in the wall because of its elongated shape or flatness ( making them able to not only show the best face but also really 'get into' the wall). Many others 'work' because of their ability to take the hard breaks and still stay together or their willingness to take up the slack and add 'heart' to the middle of the wall.
Anyone who 'employs' rocks and makes good stone walls will know you can make walls out of any of the three basic types of rock - igneous, sedimentary or metamorphic. ( as well as almost any combination of these naturally occurring geological materials ) While I cant quite say that 'I have never meta-morphic rock I didn't like', I have found most rocks, like most people, to be level bedded, cooperative, responsive and dependable, individuals who bond well with each other and are beholden to the basic principles of form and function. They make good stones.