The Stone of Erech, also known as Black Stone, was the only standing stone where the king of the mountains swore an oath to Isildur. It was depicted as being large, having possessing some six feet in radius and seemed to be spherical in shape -- half of it was underground at the time of the oath. The other half that was visible/above ground was made out to be as tall as a man. It was located on the Hill of Erech, south of the White Mountains (Ered Nimrais) in Gondor.