In Arizona’s Petrified Forest National Park, the fossilized trees look like rocks filled with minerals including quartz, amethyst and cobalt. They are dazzling—so dazzling, in fact, that visitors frequently pocket pieces despite all the signs prohibiting them from doing so. I did it myself, taking two tiny pieces of rock when I stopped at the park on a road trip a few years back.

But many people who remove the rocks end up wishing desperately that they hadn’t, often returning them in the mail. “They are beautiful, but I can’t enjoy them,” scrawl capital letters on one note. “Take these miserable rocks and put them back into the rainbow forest, for they have caused pure havoc in my love life and Cheryl’s too,” pleads another letter written on torn-out notebook paper...