You can sometimes crack a rock open with a hammer, but that's not reliable and can crack it in the wrong plane to view anything. The approved approach is to soak them in water and then freeze them--perhaps repeatedly. We'll see. I picked up quite a few opened and discarded rocks by the roadside--some of which look rather jelly-fishy. Somebody was impatient.
The strip mines are now oddly shaped lakes, with hills in between.