It was kimberlite, the molten rock in which diamonds are found, and preliminary tests had yielded a microscopic diamond. If more are found at the 80-acre site known as the Homestead property, the land could become the state's first-ever commercial diamond operation and the only working diamond mine in the United States, geologists said.
Canada currently has the only diamond mines operating in North America.
"It's once in a lifetime. You just don't find things like that every day," said Charlton, an official with Delta Mining and Exploration Corp.
The Kentucky-based firm plans to begin large-scale exploration of the central Montana site next month.
What makes this latest discovery unique is it's believed to be the first diamond ever found in Montana that was created here, rather than dropped in the state by a retreating glacier or other means. And accessibility makes the property so appealing, officials say.
Normally, it takes years to find underlying kimberlite deposits, let alone diamonds. But on the ranch site southeast of Lewistown, the kimberlite is exposed, pushed to the surface by high-speed gas millions of years ago, said David W. Baker, an earth scientist who lives about 100 miles from the site.
That accessibility, Charlton said, should cut excavation costs and lessen any damage to the prairie and the small, vacant cabin on the site.
"We're two-thirds of the way there already, so the likelihood is very good," Charlton said. "In fact, it's one of the best (sites) I've ever seen and I've had quite a few mining properties."
On the other hand, "we may not come up with anything. There's still that potential," Delta spokesman Alex Livak said.
Though rare, diamonds aren't unheard of in Montana. Prospectors have plucked the gems from stream beds and glacial valleys for years - most notably the 14-carat Lewis and Clark diamond found at the base of a steep slope near Craig in 1990.
Delta Mining got permission from the landowner to explore the site several years ago. But the firm is only now getting together the estimated $700,000 needed for larger soil samples, Delta chief executive Barry Rayment said. More tests are planned in coming months.
The company said the landowner has been cooperative, but is private. He did not return messages left at his home by The Associated Press.
Although the odds of a major find are stacked against them, Rayment believes the site, and possibly others in Montana, stand a good chance of producing commercial-grade diamonds.
Montana, Wyoming and Colorado are all known to have the right kind of geology for diamonds. Northern Colorado was home to the country's only working diamond mine, Kelsey Lake, until its closure several years ago because of legal troubles.
"There is potential (in the United States) but it's just right now all the money related to diamond exploration is being focused in Canada," said Dan Hausel, a senior economic geologist for the Wyoming State Geological Survey.
Hausel expects the search for Northern Rockies diamonds to increase in coming years, but not to the point it has in Canada, which this year became the third-largest diamond producer in the world.
"Unfortunately, Montana hasn't been prospected as much or looked at nearly as much as Wyoming, but it's got the same type of rocks underneath it," Hausel said.
Delta Mining has rights to explore five other Montana properties totaling more than 7,500 acres, but is currently focusing on the Homestead property, which has the most potential, Rayment said.
By Sarah Cooke