BISMARCK, N.D. (AP) — Hundreds of miles from the frenzied drilling in North Dakota's oil patch, an international mining company quietly sunk a hole in a bean field near St. Thomas in search of other riches buried below the prairie: diamonds.

The search came up empty according to a report published this week, but North Dakota's state geologist said he expects others to take up the quest because of the potential payoff.

"Searching for diamonds is like searching for a needle in a haystack except that this is a very valuable needle," state geologist Ed Murphy said. "That is why people are searching and will continue to search."

Geologists believe North Dakota has potential for diamond mining because rock formations in the eastern part of the state mirror those in diamond-producing regions of Canada, which now has the only diamond mines operating in North America.

"There are geologic factors that indicate we could have diamond deposits," said Timothy Nesheim, a state geologist.

The hole drilled in northeastern North Dakota was done by Kennecott Exploration Co., a subsidiary of Rio Tinto Group based in London and in Melbourne, Australia. Rio Tinto controls the Diavik diamond mine in Canada's Northwest Territories, the Argyle mine in Australia and the Murowa mine in Zimbabwe.

Few knew of Kennecott's work in North Dakota until the North Dakota Geological Survey published a report on it this week.

The company was looking for kimberlites, which are molten rock from ancient volcanoes where diamonds may occur.

Kennecott used aircraft equipped with an instrument that detects magnetic material to pinpoint the drill site in Pembina County. The so-called aeromagnetic surveys detected anomalies that appeared favorable for kimberlites, company spokesman Matt Jeschke said.

"Just because we struck out on one drill hole doesn't mean it's not a possible concept," Jeschke said. "But the concept was not encouraging enough to allow us to continue."

Jeschke said kimberlite rocks are extremely difficult to find and locating diamonds within them is even harder. The dry hole in North Dakota was a typical result for diamond exploration, he said.

"It's not unusual — 99 percent of exploration projects don't result in a mine," Jeschke said. "It was a long shot, but it always is."