Good sources for micro rock specimens are road cuts (locations where a hill has been cut or blasted through in order to construct a roadway through it), gravel beds that are being exposed by erosion, and if you live near the plains or the desert, on the ground, just about everywhere you look. Stream beds can be good, but you may have to break rocks open — the fast moving water of a stream tends to erode and break off the finer exterior details such as microscopic crystals.