In the trees around here, Rocky Mntns, it is common to find sand and/or small stones embedded in tree trunks where they've blown in the bark at some point--we get big, big winds here--then the tree grows some more and the sand/pebbles are inside the trunk. Sometimes one can see it easily, the sandy stuff, sometimes not. [I am assuming you aren't asking about the obvious, like a nail or bullet or piece of wire fence, all of which I've found over the years, though fortunately only rarely.] In any case, even sandy wood can cause sparks....
I have found stones the size of a 25-cent piece buried deep within a trunk. Does wonders for a chain by the time you realize something like that is going on.
