An American woman is being questioned in California for allegedly trying to sell a moon rock for $1.7m (£1.05m).
The woman, whose identity has not been revealed, was held in a sting operation when she showed the rock to a Nasa investigator in Lake Elsinore.
Moon rocks are considered national treasure in the United States and their sale is illegal.
The rock was recovered but Nasa investigators have not yet determined whether it is genuine.
The sting operation had been planned for several months and took place at a restaurant.
Gail Robinson, Nasa's deputy inspector general, told the Los Angeles Times: "It's possible this is a moon rock, but it has to be tested first."
It quoted Ms Robinson as saying the woman was in custody but had not been formally arrested.
Many hundreds of kilograms of rock were collected from Nasa missions to the Moon, mainly during the Apollo 15, 16 and 17 missions.
Hundreds were handed out as gifts to foreign nations and US states, and scores of those remain unaccounted for.
Two interns stole a small amount of moon rock from the Johnson Space Center in Houston, Texas, in 2002.
There have also been a number of cases of attempts to sell fake moon rocks.
And in 2009, a supposed moon rock from the first manned lunar landing, Apollo 11, given to the Dutch national museum in Amsterdam, proved to be nothing more than petrified wood.