After burning through the cash in its treasury, Aber had to bring in a partner, Rio Tinto. Rio gave Aber $10 million in exchange for 60 percent of the project. In May of 1992, Eira, who had recently graduated from college with a degree in geology, managed to get herself assigned to Aber’s field team. By 1994, the staking rush had pulled team members in so many different directions that Eira was effectively made senior field geologist by default.

In May of that year, Thomas and consulting geologist Robin Hopkins, acting on a hunch, decided to drill underwater, off an island in Lac de Gras. The drill crew wasn’t happy about it, but Eira insisted. When Thomas and Hopkins inspected the core sample, what they found took their breath away: a two-carat diamond embedded inside. Thomas slept with the core sample under her pillow and flew to Vancouver the next day. After she presented the sample to her father, all he could muster was a single word: “No!” Her response was just as concise: “Yes!”

Eira Thomas, leading the Aber field team, had discovered what was then the highest-grade diamond mine in the world, estimated to contain more than 100 million carats, worth roughly $9 billion. The stones are of such high quality that, in 1999, Tiffany & Co. cut a deal with Aber to provide the high-end retailer with some $50 million in diamonds annually through 2013. Thomas’ find was so lucrative that Aber recently spent about $242 million to purchase the world-renowned jewelry house Harry Winston.

The Canadian discoveries did something else that was extraordinary. They effectively broke the back of the De Beers cartel, as demonstrated in July 2000, when the famously secretive South African company announced it would stop its efforts to control the world’s diamond supply. In a glorious understatement, chairman Nicky Oppenheimer later explained the decision: “As new sources of supply opened up—particularly in Canada—it became evident that that role could not be sustained.”

The Canadian media glommed on to the Eira-as-heroine story. But critics contend that Thomas has gotten an inordinate amount of play out of the Diavik diamond strike and that she’s been taking too much credit. Thomas bats this aside as sour grapes. “When I tell the story, it’s always a team effort,” she says. Still, she acknowledges that some people are unhappy with the way the credit has been distributed.

At some point, however, her overexposure seems to have contributed to what would become a full-scale rift with the Aber management team. Longtime family friend Bob Gannicott, now chairman and C.E.O. of Aber, refused to make himself available for this story. Those familiar with the interpersonal dynamics say the reason is obvious—because he’s sick of talking about Eira Thomas. Says one: “The people at Aber fucking hate it when you mention Eira.”

Both Thomases would step down from Aber’s board in 2006. It’s a sore spot, something Grenville will characterize only as “a falling out.” Matt Manson, formerly of Aber and now president of Stornoway, is less generous. “Bob Gannicott goes to the Oscars now,” he says, referring to Aber’s purchase of Harry Winston, “while we still think there are more diamonds to be found.”

It was to find those diamonds that Eira Thomas founded Stornoway in 2002. Her new partner and chairman is Catherine McLeod-Seltzer, another prominent woman in the exploration community. In 1993, McLeod-Seltzer cofounded Arequipa Resources Ltd., which made a major gold discovery in Peru and was later bought by Barrick Gold for $1.1 billion in 1996.

When I ask McLeod-Seltzer whether logic would suggest that the very last people one should back in an exploration company are those that have already had once-in-a-lifetime discoveries, she laughs. “I’d rather back lucky people than unlucky ones,” she says.

Thomas explains that she and McLeod-Seltzer are simpatico because having explorer fathers inured them to the highs and lows that come with the job. They recall with humor the vicissitudes of growing up in a mining household: One day you come home from school and there’s a pool going in; “the next day, you come home, all the lights are off, there’s no heat, and your dad is saying, ‘Put a sweater on! Why do you need heat anyway?’ It’s the roller-coaster ride of this industry.”