It's not green cheese, but it might as well be. The Dutch national museum said Thursday that one of its prized possessions, a rock supposedly brought back from the moon by U.S. astronauts, is just a piece of petrified wood. Rijksmuseum spokeswoman Xandra van Gelder, who oversaw the investigation that proved the piece was a fake, said the museum will keep it anyway as a curiosity. It's a good story, with some questions that are still unanswered," she said. "We can laugh about it." The museum acquired the rock after the death of former Prime Minister Willem Drees in 1988. Drees received it as a private gift on Oct. 9, 1969, from then-U.S. ambassador J. William Middendorf during a visit by the three Apollo 11 astronauts, part of their "Giant Leap" goodwill tour after the first moon landing. Middendorf, who lives in Rhode Island, told Dutch broadcaster NOS news that he had gotten it from the U.S. State Department, but couldn't recall the exact details. "I do remember that (Drees) was very interested in the little piece of stone," the NOS quoted Middendorf as saying. "But that it's not real, I don't know anything about that." The U.S. Embassy in the Hague said it was investigating the matter. The museum had vetted the moon rock with a phone call to NASA, Van Gelder said. "Apparently no one thought to doubt it, since it came from the prime minister's collection," Van Gelder said. Researchers from Amsterdam's Free University said they could see at a glance the rock was probably not from the moon. They followed the initial appraisal up with extensive testing. "It's a nondescript, pretty-much-worthless stone," Geologist Frank Beunk concluded in an article published by the museum.