Unlike the "pet rock" craze of past years, these rocks aren't keepers.
The Arizona Daily Sun reports that some of the rocks were mailed back by people citing curses. Some were seized during inspections.
Some park visitors were caught in the act of stealing. Some of the stone was tossed out of car doors and windows before the visitors who took them reached the inspection station at the entrance of the park.
The National Park Service estimates that about a ton of petrified wood a month is stolen from the 220,000-acre park's 600,000 annual visitors. The estimate is based on the amount of rocks mailed back, picked up along the side of the road near the exits and seized.
The rocks once had been wood, part of a large forest that existed 200 million years ago. Time and sediment had slowly turned the wood to stone quartz, preserving tree rings and bark so the modern world could marvel at the beauty.
The rocks -- some pieces as big as a briefcase, others as small as a silver dollar -- make a pile weighing tons.
"Once it's removed from the original place, the damage is done," Park ranger Kip Woolford said. "There's no way to put it back where it originally came from."
The stones have been taken "out of their context," Woolford said. They no longer have scientific value and are placed on the piles.
There's no way to know for sure how much is taken. The park has more petrified wood than anywhere else in the world.
Matthew Smith, museum technician at the park, says that the museum's collection of letters sent with conscience rocks has about 1,200 pages written between the 1930s and now. Packages dropped off at the fee collection booths at the park sometimes come with letters, but not always. He gets three to six new letters a month.
The minimum fine for stealing petrified wood or pottery shards or any archaeological artifacts is $350. The price goes up the bigger the weight and quantity of the "specimens," said Nick Poulos, a park law enforcement ranger. The crime is a misdemeanor.
The typical response from visitors who are caught: "'It's just a small piece. I thought it was OK,'" Poulos said.
About two miles from the pile of conscience rocks at the southern entrance to the park, the Rainbow Forest Museum has a display of letters from people all over the world who have returned rocks that were stolen from the park.
The display is called "Mystery of the Conscience Wood." Sitting on a bench is a large piece of wood. Ranger Lauren Carter says a man came into the museum with it. He had said his father had stolen it 55 years ago. He had hidden the hefty piece of petrified wood in his truck under a stack of potatoes. There was talk of a family curse.
"It could be a manifestation of their guilt probably," said Carter, pointing to a three-ring binder underneath the display. The binder contains letters from all over the world from people who have returned pieces of petrified wood they or family members stole from the park. Many of the letters have a theme of bad luck.
Poulos said visitors may pick up the petrified wood to look at it. That's fine, as long as they put the rocks back where they found them.