Only 843 pounds of moon rocks exist on Earth, according to NASA.
Now, 30 years after it went missing, one of those precious rocks has been found among memorabilia belonging to former President Bill Clinton.
A worker found it Wednesday morning, quite by accident, inside one of thousands of nondescript boxes in an Arkansas library.
"It's a bit of a mystery solved," said Bobby Roberts of Little Rock's Central Arkansas Library System. "It's kind of like, 'where'd I leave my wallet?' "
The moon rock is one of 50 so-called Goodwill Moon Rocks brought back aboard Apollo 17 and presented to each of the states. They're unique because they're among the only NASA moon rocks ever given as gifts, says NASA's Louis Parker.
Another 50 rocks from the first lunar landing mission, Apollo 11, were also given to the states. One of those is now on display at the Arkansas Museum of Discovery, Roberts said.
A worker cataloging materials from the Clinton governorship was in the middle of going through nearly 2,000 boxes of archived documents when he stumbled upon the priceless artifact.
"He didn't know what it was until he opened the box, because the catalog for the box listed the contents as a plaque. But it made no mention of the moon rock," Roberts said with a laugh.
It's about as "big as the end of your thumb."
Roberts said it was presented to the state in 1976, when Clinton's predecessor David Pryor was governor. Clinton won his gubernatorial election in 1978.
"We're always interested when one of these comes along," said Parker, who recalled that Colorado Gov. John Vanderhoof actually brought one of the moon rocks home with him. Vanderhoof later returned it to the state.
Clinton's staff has been informed, and Roberts said they were "very excited" to hear the news.
No word on how the rock ended up among the Clinton memorabilia.
"It just got packed up, and it's just been sitting there," said Roberts.
Now, the rock is stored in a library safe, Roberts said. "But it needs to be in a museum."
CNN's Chandler Friedman contributed to this report.