The core from a stone was removed in 1958 and its existence is said to have remained largely unknown ever since, according to English Heritage, which looks after the ancient site.
But its return could help experts uncover the source of the sarsen stones used in creating the monument around five millennia ago.
During excavations in the 1950s, cracks were discovered in one of the sarsens and work undertaken to reinforce it. The core was drilled out before metal rods were inserted to keep the stone standing.
Robert Phillips, an employee from the company that carried out the repairs, kept the 108 centimeter-long core after completing his work, for many years displaying it on a wall in his office.
It traveled with Phillips when he emigrated to New York years later, but he expressed his wish that it be returned to the monument on the eve of his 90th birthday.
Two other holes were also drilled in the stone during the works, but the location of those cores remains unknown.
“The last thing we ever expected was to get a call from someone in America telling us they had a piece of Stonehenge,” Heather Sebire, English Heritage’s curator for Stonehenge, said in a news release.
“Studying the Stonehenge core’s ‘DNA’ could tell us more about where those enormous sarsen stones originated,” she added.
English Heritage said that the core presented a “unique opportunity” for its team to analyze the unweathered interior of a stone, potentially offering fresh clues.
The source of Stonehenge’s rocks has been studied and debated for decades.
Its smaller bluestones are known to have come from the Preseli Hills in southwest Wales, and earlier this year, a team of 12 geologists and archaeologists unveiled research that traced some of those stones to two quarries.
“Archaeologists and geologists have been debating where the stones used to build Stonehenge came from for years,” Professor David Nash from Brighton University said. “Conventional wisdom suggests that they all came from the relatively nearby Marlborough Downs but initial results from our analysis suggest that in fact the sarsens may come from more than one location.”
The 4,500-year-old site was rescued from disrepair after landowner Cecil Chubb gave it to the nation on October 26, 1918, allowing crucial maintenance work to take place.
It has remained a hugely popular tourist destination ever since, intriguing visitors and researchers alike.
One of Phillips’ sons, Robin, said he hoped that the other two cores are also discovered and returned.
“It would be fascinating to know where the other two cores went, or indeed if there any other missing pieces out there that might be returned one day,” he said.