She promised the moon, for a sky-high price. He wasn't buying.

A woman who tried to sell what she said was a rare piece of moon rock for $1.7 million was detained when her would-be buyer turned out to be an undercover NASA agent, officials said Friday.

The gray rocks, which are considered national treasures and are illegal to sell, were given to each U.S. state and 136 countries by then-President Richard Nixon after U.S. moon missions and can sell for millions of dollars on the black market.

NASA investigators and Riverside County sheriff's deputies detained the woman after she met Thursday with an undercover NASA investigator at a restaurant in Lake Elsinore, about 70 miles southeast of Los Angeles, the sheriff's office said. The investigation was conducted over several months.

Authorities swooped after the two agreed on a price and the woman, whose name has not been released, pulled out the rock.

NASA planned to conduct tests to determine whether the rock came from the moon as the woman claimed.

"We don't know if it's lunar material," said Gail Robinson, deputy inspector general at the space agency.

Joseph Gutheinz, a University of Arizona instructor and former NASA investigator who has spent years tracking down missing moon rocks, said a lunar curator at a special lab at Johnson Space Center would carry out the testing. Among the substances the rock could contain is armalcolite, a mineral first discovered on the moon and named for Neil Armstrong, Buzz Aldrin and Michael Collins, who was on the Apollo 11 lunar mission crew.

The woman has not been arrested or charged. It was unknown how she obtained the rock or came to the attention of NASA.

Gutheinz said the woman could face theft charges if the rock is genuine, or fraud charges if it is not.

About 2,200 samples of lunar rocks, core samples, pebbles, sand and dust _ weighing about 840 pounds _ were brought to Earth by NASA's Apollo lunar landing missions from 1969 to 1972. A recent count showed 10 states and more than 90 countries could not account for their shares of the gray rocks.

Gutheinz said most purported moon rocks offered for sale on the Internet are bogus, though authentic moon rocks can be purchased if they came to Earth in a meteorite.

NASA houses 70 percent of its lunar rock and soil samples at Johnson Space Center, and another 14 percent are in New Mexico. The rest are either on loan for study or display _ or are unaccounted for.

In 2009, the Rijksmuseum in the Netherlands confirmed that one of its rocks was a fake and not an artifact collected by the Apollo 11 crew.

A rock presented to Honduras was recovered in a 1998 NASA sting after a Miami collector offered $5 million for it.

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Associated Press writer Thomas Watkins contributed to this report.