This secret was taken to the grave.

Archeologists believe they have finally uncovered the origins of what is believed to be the oldest gravestone in the United States — which belonged to an English knight who died nearly 400 years ago.

The tombstone, from 1627, was erected at the Jamestown settlement following the death of Sir George Yeardley, a colonial governor of Virginia.

For decades, experts have been scratching their heads trying to determine the origins of the stone.

However, according to a new study published in the International Journal of Historical Archaeology, the stone was not North American.

The study, titled “Sourcing the Early Colonial Knight’s Black ‘Marble’ Tombstone at Jamestown, Virginia, USA,” posits that the stone was hauled across the Atlantic.

“In seventeenth-century Virginia, USA, one of the ways affluent English colonists exhibited their wealth and memorialized themselves was with engraved tombstones,” the study states. 

“Wealthy colonists in the Tidewater region of the Chesapeake Bay at this time preferentially selected black ‘marble’ for their gravestones that was actually polished, fine-grained, black limestone.”

Researchers used fossils of single-cell organisms contained in fragments of the gravestone and identified six species that did not exist in North America — meaning the stone had to have been imported from Europe.

The stone was most likely transported from Belgium before it made the months-long journey to Jamestown.

“This supports the conclusions above for transatlantic trade routes from continental Europe to Jamestown,” the study said. “These were undoubtedly not direct, but through London.”

The stone was quarried and cut in Belgium before it was shipped down the Meuse River and across the English Channel to London. 

There it was further carved, the brass inlays were installed, and it was loaded onto an American-bound ship as ballast.

The stone is believed to belong to Yeardley, who was leading the nascent colony at the time of his death.

“Assuming the knight’s tombstone was George Yeardley’s, then it is the oldest black ‘marble’ tombstone in the Chesapeake Bay region, and may be the oldest surviving tombstone in America,” the study said.

“It is the only known tombstone in the English colonies with engraved monumental brass inlays.”