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Head bent, eyes fixed on the ground, Doug True prowled along the river’s edge.

After a few moments, he clawed at a rock embedded in the sandy soil, lifting it out of the ground with the curved metal prongs of a long-handled beachcomber’s scoop.

Part of the rock’s dark blue, scaly surface had a web-like patina of white. A bit of broken rock face, a fractured chip of the rock’s surface, offered a window, a waxy-looking clue to the agate hidden within it.

When True hunts along the river, his pursuit involves a dogged perseverance.

In late September, with the sun warming to T-shirt weather, the Yellowstone River west of Miles City ran low along the gravel bars. Graying hulks of uprooted tree trunks punctuated the long stretches of river rock.

Higher up, thickets of scrub brush merged into stands of cottonwood.

“Hunting agate’s a crap shoot,” said True, the president of the Billings Gem and Mineral Club.

“You can go out one day and hunt and come back with three or four pounds of nothing. The next day, you can go out and find 50 or 60 pounds of really great stuff.”

Montana agate can be found along the entire length of the Yellowstone River, but the stretch between Forsyth and Sidney contains some of the most highly prized deposits.

Agate is a form of chalcedony, which is one of the many varieties of quartz.

While some type of agate can be found in nearly every state, Montana agate is known by rock hounds around the country for the beauty of its designs and its hardness, which allows the agate to take on a fine polish.

True began hunting agates along the river soon after he moved to Billings in 1969.

Some of his specimens, along with more than 50 displays by other club members, will be at the upcoming Billings Gem and Mineral Club’s annual show on Oct. 16 and 17 at the Billings Hotel & Convention Center.

Through the year, the Gem and Mineral Club offers fieldtrips, which give novice rock hounds a chance to discover the beauty of nature that can be found in Billings’ backyard.

Prime agate-hunting season begins in the spring, after ice jams and winter’s snow melt have churned up a new set of agates to find.

But, even in fall, at the tail end of the season, True plies the river in a 16-foot flat-bottomed aluminum jet boat geared for river running. For him, agate-hunting season has no cut-off date.

“I’ve dug agate out of the river with a screw driver when it was frozen,” he said. “Does that tell you something?”

He spends about 50 days a year out in the field, collecting various types of rocks in different parts of the country.

It’s a hobby that he honed in his youth, when he sifted through the tailings of tourmaline mines near his childhood home in Southern California. His father, an archaeologist and anthropologist, encouraged him to look for something whenever he got out the car, searching for rocks in the desert and seashells along the coast.

“It’s just the hunt,” he said. “You never know what you’re going to find.

“I’m just as happy looking for a nice piece of petrified wood or digging in a mine dump looking for pyrite crystals or quartz crystals. I think it’s just the hunt and what you might find that you haven’t ever seen before.”

In February or March, True heads for the river as ice jams break up.

“Those big chunks of ice just go through those gravel bars like a bulldozer and completely open up the rock,” he said. “If that happens, then there’s a lot of great hunting for a while.”

His schedule revolves around collecting.

When the water rises with the snow melt in early to midsummer, it washes the rock. But, if the river level goes down slowly, it leaves a coat of silt and algae that bakes on the rock’s surface, making it tough to spot the agates.

“This year has been exceptional because all the storms and thunderstorms and hail has washed a lot of the rock off,” he said.

The Yellowstone’s agate was formed 40 to 50 million years ago, spawned by massive volcanic upheaval in a huge region centered around Yellowstone Park.

Volcanic ash covered forests. Silica dissolved into ground water and seeped into cavities in the igneous rock, forming the fine-grained crystal structure known as chalcedony.

Different stretches of the river provide different concentrations, sizes and colors of agates.

Agates with a reddish exterior are more common in the far eastern portions of the river, around Terry, Glendive and Sidney. Blue colors are more common from Forsyth to Miles City.

Any of them have the potential to be highly prized scenic agates with dendrites, inclusions that look like multi-branching tree-like patterns.

The orange and red colors of the dendrites come from iron oxide, while the blacks and browns come from manganese.

As True worked his way along the gravel bank, he rejected some rocks as

“mud agates,” mixtures of jasper and agate or other impure combinations of minerals. The worthless rocks are sometimes called “leaverites,” short for “leave ‘er right there.”

True was loaded down by canvas tote bags filled with agate when he headed back to the boat after a few hours combing the gravel bar.

He slung the tote bags over the handle of his metal-digging tool and balanced the handle across his shoulders.

Some of the agates were bigger than softballs, but their “windows” offered only a hint of their potential beauty.

“If it’s agate, it comes home. Big, small, or otherwise. Some of the smallest rocks have the most beautiful designs and things in them,” True said.

He brings a screwdriver to dig with, but hates the idea of carrying a hammer.

“You go along the river and see where somebody’s dug up a rock, and the first thing they do is break it open,” he said.

“In most cases, it could be a very good rock, and it’s no good anymore.”

The best way to learn to hunt for agates is to join a field trip with an experienced rock hound, True said.

When True takes a newbie on the river, he’ll spot an agate and then ask the novice to spot the same rock, narrowing the search down to about a 4-foot radius.

“Your eyes have to just get adjusted to it,” said True’s wife, Jeanette, whose own rock-hunting field trips are limited by her asthma.

The couple, married for 44 years, travels to about 15 rock and gem shows each year to sell minerals, specimens and jewelry.

They spend January and February selling items through a rock shop at Quartsite, Ariz., a town 80 miles north of Yuma and a Mecca for rock hounds because of its many gem and mineral shows.

In February or early March, when the ice breaks up along the Yellowstone, they return to Montana.

In June, they head for central Oregon, hunting mainly for petrified wood, his wife’s favorite. Afterward, they spend another month hunting agate along Oregon’s beaches.

True has been known to start upstream on the Yellowstone at daybreak and not come off the river until dusk.

Jeanette, a slightly less-avid rock hound, sometimes digs for a while and then reads a book. She appreciates just being out on the river.

“It’s just so peaceful,” she said. “You get a lot of God time, too.”

Contact Donna Healy at dhealy@billingsgazette.com or 657-1292.

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