I recently had a dramatic illustration of this. My wife and I, driving through Arizona, stopped at a"collector's shop"in Tucson, where stones and minerals of many kinds were on display.In the course of the visit, we were taken into a small room where rocks were laid out on shelves.They were quite ordinary-looking rocks.Had I seen them on some hillside, I would not have given them a second thought.Then the man closed the door so that the room was in total darkness and turned on an ultraviolet lamp.