The stone "core" was removed during archaeological excavations in 1958 and its existence remained largely unknown for six decades, but it now joins English Heritage's collection of more than 500,000 artefacts.
Experts now hope that its return may help uncover the source of the stones that form much of the monument.
The core was removed during the raising of a fallen trilithon, a group of two upright stones and a third across the top, in 1958.
It was drilled as part of measures to use metal rods to reinforce one of the vertical stones, which was found to be cracked, and the repairs were masked by small plugs from sarsen fragments found during excavation.
The work to drill three 32mm holes horizontally through the metre-thick stone was undertaken by Basingstoke diamond-cutting business Van Moppes.
Company employee Robert Phillips kept one of the Stonehenge cores, which he gave pride of place to in his office, and then took it with him when he left the firm and later emigrated to the US.
On the eve of his 90th birthday, he expressed a wish that the fragment of the world famous prehistoric monument be returned to English Heritage.
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His sons Robin and Lewis travelled to Stonehenge last year and presented it to the conservation charity, which looks after the ancient stone circle.
English Heritage does not know if the other two Stonehenge cores survived and is urging anyone involved in the excavations during the 1950s, or whose family was, and has any information on the cores to get in touch.
Heather Sebire, English Heritage's curator for Stonehenge, said: "The last thing we ever expected was to get a call from someone in America telling us they had a piece of Stonehenge.
"We are very grateful to the Phillips family for bringing this intriguing piece of Stonehenge back home.
"Studying the Stonehenge core's 'DNA' could tell us more about where those enormous sarsen stones originated.”
Lewis Phillips said: "Our father has always been interested in archaeology and he recognised the huge importance of the piece of the monument in his care. It was his wish that it be returned to Stonehenge.
"We are all delighted the core has come home, particularly as it is now being used to further important research.”
Anyone with information about the other two cores can get in touch by emailing firstname.lastname@example.org.