The stone, also known as a bullaun, around 10 inches in diameter and engraved with an early Christian cross, was found to fit perfectly into a large rectangular stone with a worn hole located at the base of a standing Christian cross on the Isle of Canna in Scotland's Inner Hebrides, the BBC reported.
"Stones like this are found in Ireland, where they are known as 'cursing stones', but this is the first to be discovered in Scotland," Katherine Forsyth, based at the University of Glasgow, said.
"They date from the early Christian period but have continued to be used by pilgrims up to modern times," Forsyth, an expert in the history and culture of early Celtic-speaking peoples, said.
"Traditionally, the pilgrim would recite a prayer while turning the stone clockwise, wearing a depression or hole in the stone underneath."
While a number of the bowl-shaped lower stones had been found elsewhere in Scotland, this was the first discovery of a top stone, she said.
"This exciting find provides important new insight into religious art and practice in early Scotland and demonstrates just how much there is still to be discovered out there."