Open this photo in gallery Curling coach and spy Jules Owchar of Team Newfoundland and Labrador watches during the Grand Slam of Curling in Calgary on Thursday. Todd Korol/The Globe and Mail

Jules Owchar sits on a folding chair, behind a scoreboard at a curling game, spying for his team. He looks through black-and-silver Bushnell binoculars at what’s happening on the other sheets of ice, where his team’s opponents are playing. He looks, takes notes. Looks, takes notes. He walks a few feet to his right – now he’s behind another game’s scoreboard – for a closer peek.

Mr. Owchar is scouting stones. Every curling rock has its own personality – and a letter and number on its coloured handle making it identifiable. Some stones consistently slide faster than others. Some curl more than others – bend, that is, to the inside or outside of the sheet as they make their way down the ice, because the athletes intentionally threw them a certain way. Some are just wonky. And, after all that, a rock’s behaviour changes depending on everything from the ice to the shooter to the time of day.

Mr. Owchar watches every stone, during every game, at every elite tournament. He jots down intel in a white vinyl Labatt folder he’s had since the 1980s. He makes charts detailing every rock’s characteristics, then shares his analysis with the team he’s coaching. The information is crucial for pro curlers to gain an advantage over competitors. It gives them a chance to adjust quickly to every rock’s idiosyncrasies. It is a sophisticated dark art.

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“When I first started, it was a secret,” Mr. Owchar says between games at the Pinty’s Grand Slam of Curling: Humpty’s Champions Cup in Calgary this week. That was about 30 years and hundreds of thousands of stones ago.

Now this cat-and-mouse strategic manoeuvre is commonplace at elite bonspiels. But just as some curlers are better than others, some scouts are superior. You want Mr. Owchar on your side, even if he seems like a bit actor someone found roaming West Edmonton Mall.

“I just give the trend and then it is up to the boys,” he says. Sometimes that means his team – he’s backing Brad Gushue’s rink these days after siding with Kevin Martin for three decades – tosses out his recommendations when they get a chance to throw the rocks themselves. Feel trumps all.

Open this photo in gallery Jules Owchar watches every stone, during every game, at every elite tournament. Todd Korol/The Globe and Mail

The invitational Grand Slam of Curling tour travels across the country with 80 game stones plus two spares. Each tournament usually involves five sheets of ice. Each sheet hosts 16 stones – a set of eight with red handles for one team, a set of yellows for the other. Each of a team’s four players throws two rocks during an end, which is like an inning. Each player wants rocks that “match” – curling-speak for stones that exhibit similar behaviour. The skips select the best of the bunch because their shots are often the most complex. Their teammates get the leftovers.

These stones are made of Common Green granite with Blue Hone granite inserts. The Grand Slam’s collection costs about $51,250 and should last about 20 years in the hands of elite curlers.

But elite curlers are stone snobs.

This is the third set the tour has used in the past four years, according to Greg Ewasko, one of the tour’s two ice makers, who must keep the stars pleased.

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“The curlers were tired of playing with the original set because they were finding too many inconsistencies,” he says. The current roster of rocks has soothed some curlers, says Mr. Ewasko, who chooses words with the touch of a diplomat.

“The happy outweigh the unhappy.”

He admits he is tempted to mess with the colours, letters and numbers that identify rocks, just to see if the finicky curlers and coaches would notice. “Yeah, there’s sometimes, but we don’t,” he says. “That’s career-ending.”

Jennifer Jones is a brand-name curler. Canadian champ, world champ, Olympic champ – she’s been all those. She’s among the athletes who would notice any funny business.

“The best stone matchers are usually some of the best players,” she says.

Her team didn’t vigorously chart rocks until after the 2005 Canadian championship. They won, but Ms. Jones had to make a tricky shot with the game’s last rock to score three points and claim the title.

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“I didn’t have very good stones,” she says. “It was after 2005 that we really focused on making sure we had good rocks.”

Open this photo in gallery Glenn Howard in action during the Grand Slam of Curling in Calgary. Todd Korol/The Globe and Mail

And then there’s Glenn Howard, another player who is curling famous. He and his brother Russ pioneered charting rocks. “We kind of went all anal about it in the early eighties,” he says.

Their competitors used to copy which stones they threw because they were the best at it. The Howards responded with counterintelligence tactics. They would throw random rocks in practice, rather than the ones they intended to play in the game, to throw off the opposition. They put tape on the stones’ numbers and letters so spies such as Mr. Owchar couldn’t track their rocks. The tape was effective, but it was also the reason the rules changed, prohibiting curlers from defacing stones.

Mr. Howard’s team evaluates just their rocks after each game, collecting notes in black memo pads. They lost Thursday afternoon, throwing the yellow stones on sheet E.

“We didn’t love that set,” Mr. Howard says.

The rocks that curlers deem inferior usually end up in the hands of the leads, the players who throw the first two stones for the team. Mr. Owchar says the pecking order is a reflection of their skill.

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“The leads are so good now – they get the bad rocks, but they can adjust so well.”​