The best place to collect your rocks is from outcrops. Outcrops are the visible parts of the rock formation that makes up an area. For instance, a rock that is not attached to the underlying rock formation is not an outcrop. A boulder is not an outcrop. A cliff face is an outcrop.
Rock formations exposed by an excavation are outcrops.
Don't collect gravel from a driveway . . . If you have a rock hammer you can use it to break a small piece off a larger in-place boulder. This is the best way to ensure that it's in place. A topo map shows hills, valleys, plains, mountains, cliffs, and streams. From a topo map you can determine how tall a mountain is, how steep a cliff is, and how long a river is.