Ranger Chuck Dorn has seen just about every technique there is for swiping petrified wood. Some sly, most not.

In his seven years at Petrified Forest National Park, he has known tourists to hide the fossilized wood under handkerchiefs, in plastic foam cups, in their pants--just about anywhere it fits.

"The baseball-, softball-size ones get collected up pretty quick in sites where there's a lot of visitors," Dorn said. For larger chunks, thieves sometimes hike behind hills and hack a piece off.

In fact, so many tourists want the natural mementos that one study suggested as much as 12 tons of the petrified wood--enough to fill four full-size pickup truck beds--vanish each year.

The problem is the worst in the park's popular tourist hangouts, such as the Crystal Forest, where only large chunks of the tan, purplish and pink wood remain. Piece by piece, visitors have swiped smaller nuggets, leaving barren patches between the larger hunks of the 225-million-year-old fossilized wood.

Hauling off the wood is illegal and at least marginally risky--violators face fines of at least $250.

In 1998, rangers cited 64 people, fining them $23,850. In one case, Dorn said, a Southern California man was fined $2,500 after being caught with 440 pounds of petrified wood shoved under every seat of his van.

The park's seven rangers sometimes search the vehicles of suspected thieves, but they can't possibly query everyone who visits the 92,000-acre forest.

And while uniformed volunteers and other security measures--like asking visitors to promise in writing that they won't steal--have helped reduce theft, nothing is likely to stop it altogether.

"The only 100 percent solution is to keep people away or have an armed park guard and that's not feasible," said Greg Caffey, the park's assistant chief ranger. "It's not like a big evidence locker. We have to provide for the enjoyment of the visitors."

Of those who get away with their crime, some suffer a guilty conscience. In September alone, 25 pounds of rocks were mailed back to the park--some with apology notes, Dorn said.

But pieces that are returned or recovered can't go back to the park grounds. It is almost impossible to know what part of the park a piece came from, and the wood can foul up scientific research if it is dropped in the wrong spot.

The wood has its origins in a volcanic eruption that researchers believe knocked down trees and washed them into low-lying areas, where they became waterlogged and sank. The volcanic ash turned to silica in the water and was absorbed into the cells of the trees, turning to quartz.

Only 10 percent of the region's wood is in the park, and pieces obtained legally from private land are readily available in shops in and around the park. Still, that doesn't stop visitors from taking souvenirs.

"You see them with a Styrofoam cup in their hands. They have four grandkids, so they have four pieces. They'll stop and see a pretty piece and they'll comparison shop. And they'll throw one back," Dorn said, laughing.