A former Nasa investigator is trying to track down dozens of tiny lumps of Moon rock that disappeared after being given away as goodwill gifts.
Very few legitimate pieces of Moon rock are in circulation
Former US President Nixon gave away 135 rocks in the 1970s but only 24 can be accounted for, Joseph Gutheinz told the BBC's Newshour programme.
Of the 135 Moon rocks, he said, probably about half had either been stolen or lost or soon would be.
Moon rocks are thought to be worth up to four times as much as diamonds.
"We have been having a world of hurt trying to find these things," said Mr Gutheinz.
The 1.1-g (0.04-oz) fragments were given to friendly foreign governments.
They were mounted in a clear sphere on a wooded plaque bearing the nation's flag.
The last fragment to come on the public market, which weighed less than 0.3g (0.01oz), was sold at auction for more than $400,000 in 1993.
The American Apollo missions collected 382kg (842lbs) of rock, which remains the property of the US government.
Mr Gutheinz has contacted a number of foreign embassies, museums and universities in a bid to find the missing material.
But poor security and tracking means much of it has vanished from public view.
No rocks for Astronauts
One rock given to the Honduran government disappeared in the 1990s without anyone realising it.
It was finally recovered in a sting operation last year, organised by Mr Gutheinz, after it was offered for sale for $5m.
The dealer involved said he had bought it from a Honduran general for $50,000.
Another rock given to the people Malta was stolen from a museum.
"There was no monitoring, no surveillance and no security," Mr Gutheinz said.
He is angry that the US government gave the pieces of Moon rock away without consideration for their safety.
"We gave these Moon rocks to some of the biggest dictators in the world," he said.
"There was no tracking system and yet our own American heroes, the astronauts that went to the Moon, are not permitted to own a Moon rock," he added.
"The idea that somebody would steal them, would pocket them, something that belongs to the people, I find wrong and I'm incensed by it," he told the BBC.