Deeply rooted in Celtic tribal tradition.
Back in the day tribal folk were tied to spirit and it was essential to have connection and blessing from the ancestors to move to the next stage, especially regarding marriage.
It was believed that an oath given over a stone or water was more binding, hence the oathing stone.
The couple would both place their hands on the stone and pledge their oaths. This would be witnessed by the tribe, the chief and provide a bridge for all the ancestors who went before.
At the end of the ceremony the stone would take its own journey being tossed into the loch or over a cliff to the sea.